I actually read it all.
And I still think they need a 'braking distance' and a larger 'turning circle' when moving at speed.
It is Inertia.
The universal law that dictates about objects in motion involving their mass.
http://en.wikipedia.org/wiki/Inertia

In short the energy has got to go somewhere, either it's bled off into the ground (braking), also into the graound wider circle, into an object (ricochet turn).

Agility and conservation of energy are important to visually realize but keep in mind that space marines are on a scale an order of magnitude larger than normal people. Their most agile tight turn is going to be necessarily larger and displace proportionally more energy to perform than a normal human being. Scale in this case matters proportionally.

But you don't take into account the fact that a Space Marine can take the strain and has the strength to counter that inertia. We would break our bones and fuck up our ligaments if we tried to do something like that with the same amount of inertia. But Space Marines have physiques that are way more resilient and tough than a normal human being, their reactions are quicker, they wear a suit of armour that basically gives them a second group of muscles and even more protection. Those guys are insanely capable of pulling out feats we can only imagine.
Not to mention that a Space Marine is a trained supersoldier. TRAINED. This implies training in how to move, and all kinds of techniques to get the best out of their bodies and limit to the bare minimum their weaknesses.
A Space Marine won't break a run by just stopping mid-charge or mid-sprint to face another enemy. They use the inertia in their advantage rolling forward, or over, to jump back up facing the enemy and potentially gaining a better position to counter this enemy. Or redirecting their charge to bash the shit out of the enemy, or to just ignore him until a better position is gained. Also their martial approach is designed to take advantage of the Space Marines physique. Not to mention how insanely quick they are when they want, fluff wise, meaning that even if they do stop mid-run and have a breaking distance, this would be to normal humans insanely fast considering the speeds they can go up to.
Space Marines are quick and resilient. They know how to move in their physiques better than a regular Eldar or human would. They have a set of armour that enhances their already superior abilities. For a Space Marine to break a run he would do so but with less difficulty than some expect and with much more speed than a human.
Even if a breaking distance, although i would call it a breaking moment, is implemented it should be something that takes 1 or 2 seconds at most. And it should show the SM just putting one step more and holding all that inertia for that amount of time until he regains full control. He wouldn't have to run a single meter to counter that inertia, he does so by just planting the next forward step hard and countering that inertia with his awesome power.

They are not Movie Marines!
Not even a Space Marine can make one-to-two tons of weight, travelling at 30 miles or more, come to a dead stop without some form of breaking distance. No matter how good their training.
(Terminators could, but that's because you can't run in Tactical Dreadnought Armour.)

Now you have mentioned them using other methods for shedding inertia, like a roll or a jump.
But that would have to be player activated, and what we've been talking about is something the model does automatically if you don't make them actively shed inertia.

What I have been suggesting is that an SM or CSM skids a couple of strides (about equal to their height) when stopping from running at their top speed/charging. They could even take 2-3 steps to shed it.
And when moving at this top speed they have a larger turning circle than Eldar or even a normal human would at their top speed.

I am suggesting this as it not only helps balance out the Factions, both current and prospective, but also strikes a balance between gameplay, feel in-game, and reality.

You can do a little charge in termi armor, aboit half the speed a norm sm could run at, bust still bloody fast.

Lmao, where do you get the 2 or 1 tonnes from? Deathwatch rpg book, page 165, table 5-13: armour. Astartes Power Armour 180kg. Artificer Power Armour 100kg. Dreadnought Tactical Armour 400kg.
An Astartes in full Power Armour weighs around half a ton, even less. Bear in mind SM aren't encumbered by normal Power Armour, meaning the suit itself counters the strain and weight of said suit. Whilst augmenting the Space Marine in many aspects, he becomes quicker (adds one more movement point whilst wearing said armour) augments strength and general resilience. The armour effectively absorbs many of the strains related to movement, including breaking at mid-run.
Space Marines aren't trained to be only strong or resilient. They train to enhance their potency or power, all the conditioning and alterations they undergo are geared towards this. To find a balance between strength, speed and stamina. They think and react at lightning speeds. They aren't superhumans for nothing.
As to speed it is more like 42 kph, whilst 30 mph is about 48 kph. (Again gained from the movement table from the Deathwatch rpg book).
The book i'm taking all this info from is the closest thing we have to "true" values. As the novels vary from author to author, due to artistic liberties. Not to mention that many pieces of art are symbolic more than realistic, don't even get me started on fanart or the description of them made by awed mortals.
Basically: you exaggerate the weight of an astartes, running speed isn't as fast but still double a professional athlete, you don't factor in how the armour absorbs a lot of the strain as it makes the user not only not feel the weight of the armour but also effectively makes them stronger and tougher, couple this with an enhanced physique (not just bigger, but better. Better at supporting strains for example) and superior reaction speed. Than you have something that is capable of breaking his run even at such speed.
It would be very stupid to create such a being without making him able to support the strain from performing a sudden halt, even at full speed. Otherwise you will have marines falling over or injuring themselves by trying to do so at full speed.

Hmmm reading these posts is giving me a hard time to wrap my head around the SM and power armour's movement. It should have a bigger inertia, due to a bigger mass, but then again, power armour is... powered, and is effectively an extension of the SM's body.

I think we need a physichs expert, rather than a lore expert, to answer this question.

Also in biology and other fields. Astartes bones for example have a different composition and density from standard human ones, this is not even regarding size difference. But a educated guess is that the bone composition of an astartes is way more capable of supporting strains than a human one.
Not only do we have to see how much inertia he generates by running but also what kind of body composition an astartes has and the role of the PA. Muscle tissue and ligament composition and characteristics are also paramount.

indeed, I would take bone/muscle resistance out of the equation and assume they can take whatever tension required to make certain movements (unless it's something very absurd).

But the main point should be that SM are not natural beings, they are an artificial beings. So a mind (emps) made them able to do crazy shit a normal humanoid could never accomplish, and the enhancement is not only physical but mental too. The crazy amounts of pain they can endure for example. The SM is designed to pull off stuff other things can't.
I'm not directing this to you, but just adding another point for people to see my point.
SM are not enlarged humans. Make a human being larger, without altering his physiology but just make him bigger until he has the same size of a regular marine. That still isn't an SM. SM are way better than just larger humans, they are stronger, more resilient, have superior memory, bone structure that better protects and allows greater feats etc. They are effectively not even human anymore, they are one artificial step ahead in the evolutionary curve.
Blend in PA, that they where originally teenage boys selected for being more capable physically and mentally than base humans, extensive training including techniques to maximize movement and maneuverability whilst wearing and not wearing power armour.
When you fully comprehend what they are (i haven't even gone into their secondary organs and other stuff), your mind gets blown and they can break their run in a small amount of time and space.