Just to clarify. Eldar took no bonus damage. They took full damage without mitigation.

Everyone else mitigates 9% damage from 80 pen weapons.

Now i question myself...
Eldar toughness 80?
Bolter pen 100?
I can't check for myself atm, would you...?

That should change nothing for Eldar. Pen doesn't give bonus damage, it just allows you to bypass toughness to a point.

If anything, raising base penetration to 100 will nerf everyone but Eldar.

Bolter penetration is currently 80 if that's what you are asking.

Oh my gosh... i was wrong the whole time i thought eldar would get a buff from that, as they were reciving bonus-damage because of higher pen / lower toughness. My Bad, excuse me!

In that case, leave it as it is! Fix lag, so my shots actually connect with eldar and i am fine

My main question is, if we buff Eldar toughness to 100, but each weapon that was 80 pen goes to 100 pen at the same time. What is the net gain?

If pen over toughness doesn't add to damage than dropping Eldar EHP because they went from 80 to 100 while each weapon is also getting boosted means it'd be a straight nerf.

Right now vs a Bolter a Dire Avenger has 332 EHP, but if they increase DA to 100 Toughness, and Bolter to 100 Pen, but drop Eldar by 20 health.. than I just have a EHP of 312.. a nerf.

Right now vs a Plasma Rifle the same Dire Avenger has 332 EHP, but again.. since pen over toughness doesn't add to damage it would just be a straight nerf in HP

As for the damage boost that some Eldar weapons would see, is this not the same damage boost their 80 pen weapons would see vs our classes running armors + naturally tougher classes (DR, SS and FD)

So I vote to keep Eldar HP exactly the same but bring them up to 100 along with all the pens.

The fact is that Eldar will do more damage than actually because the pen/toughness gap is reduce. Other faction gun will have the same result, but damage done by Eldar will go up because negating the 20pts of toughness damage mitigation actually in game.
So Eldar will kill faster so they also have to die faster. Or if we don't touch Eldar traits all others factions get an armour or hp boost of more or less 10%

Right, and I get all of that, but I think the weight of the changes are not being taken into account.

So the primary weapon for each class is dominant way they kill.
So
-Warlock- Witchblade: not affected by this
Destructor: (for caster build) is not affected by this. (I know it's not used much but one day there
might be more than just me that use it)
-Scorpion- Chain Sword: will benefit from this
-Banshee- Power Sword: not affected by this
-Avenger- both guns affected by this
-Hawks- two of three guns affected by this
-Dragons- not affected by this
-Reapers- one of three guns affected by this

So 6 primary weapons are effected by this which is where this will predominately be noticed.
7 primary weapons that are not affected by this. (6 not including Destructor)
If you break it down by classes primary weapon affected you have 4, but 2 of those are only partially affected depending on weapon choice. Classes not affected by this is 5, 2 of those again depending on loadout.

So basically you have half of our classes primary weapons not affected by this but yet they want to apply a nerf to the entire race's EHP. This is what doesn't make sense to me. So it is a fair tradeoff for a small pen buff to only some weapons, and yes I know it affect the pistol which actually needs a buff to begin with to brought up to par with other factions pistols, and then there is the knife which will assist but I suspect without doing the math it is not going to make the knife kill any faster than it does already, and if not this change means nothing to them.

I'm not sure why they need to 'normalize' anyway, I think current is fine in the regards for representing weaker Eldar and fits better to TT flavor of not as tough but as much health as Marine.

My guess is it's to help the weapons most commonly found in Presets(which are apparently somehow going away but I don't really get, you're still going to have "starter gear" on a new player that will probably be the same as presets now) to compete with the now flooded game of veteran players with all gear unlocked.

My recent project of a new Rank 1 LSM has taught me the aggravations and futilities of a Chainsword preset. If I had a dollar for everytime I barely lost a fight because Chainsword doesn't cut through armor enough like a Power Sword+ I'd be eating Arby's all week for free.

The problem isn't your weapons because your enemy already got 10o toughness but the effect of SM and Orks weapons loosing 20 points in damage mitigation.
Let me explain it with an example,the numbers will be false but the fact is the same:
-let's say before toughness adjustment your max was 120 it will then become 140, so all weapons between 121 and 140will be less effective and will suffer from a damage mitigation. So to rebalance Eldar have to loose something or there will be no more glass canon concept.
And don't try to convince me that Eldar are only glass, my personnal experience tell me the contrary.