I don't have an exact example, but I have an allusion, will that do?

So, in PS2, which is probably the closest game to EC that exists right now, most of the weapons are either purchased or need to be tediously grinded for over a couple weeks. PS2 skirts the boundary of being called PtW because of that - people do complain, both critics and players, about the costs of the guns and how difficult they are to obtain, but just the option of grinding drowns out any serious PtW accusations.

Now, if that grind option didn't exist, I think things would get really messy. For instance, there have been a couple times when a single weapon type has become incredibly important in large PS2 battles, but arose because of exploits in situational advantage, not power imbalance. The Annihilator rocket launcher was a good example - it had pathetic damage, but very long range and a lock on capability, and thus when used by many people at once in a large strikeforce it became an unstoppable weapon against vehicle zergs. The gun was balanced individually, but suddenly it was changing the entire dynamic of the game due to large scale cooperative use, and for a while it made it nearly impossible to run as aircraft until it was nerfed and people figured out counters.

Now, imagine if that gun had not had a grind option, but had instead been payers only, or worse, founders exclusive. That balanced but game-altering weapon massacring aircraft in every battle would have become a luxury for paying players at the expense of every non-paying player in the game. Even a quick nerf wouldn't stop the backlash, and would have to come at the expense of what would otherwise have been a balanced and teamwork-requiring game mechanic had it been available to everyone. Not a good situation for devs or for players.

That's my view, anyway. I hope it puts the concern in better perspective.

But that's not what's happening here.
The standard Sunfire Pattern Plasma Gun will be available to all, you just have to earn it.
But the Apollo Mk1 you buy. And the difference between the two? It seems like the Apollo's Plasma will burn hotter, doing more damage but this damage increase also works when the weapon blowsback/damages the user, which is a chance with all infantry Plasma weapons.

On the TT infantry Plasma weapons have the 'Gets Hot!' rule which means you have a 1 in 6 chance of causing an injury to the weapon's user, which considering that most individuals in TT have only one wound, means it can be fatal.

You are discussing a tradeoff in balancing, which is the principle all PS2 weapons are built on. There are many, many other AA locking rockets in PS2, but only one affected the game with that degree of impact - the Annihilator. It was a simple balance tradeoff with between range and damage that did made all the difference. Other rockets had better damage or reloads or whatever but only the annihilator scaled with guild coordination in an exponentially powerful way. And what's more, there's pretty much no way that outcome could have been predicted, because the rocket was balanced or even underpowered in low-population testing.

Random example for EC. Say there's a founders/purchase only relic sniper rifle that has crappier than usual damage, a long reload, but slightly longer range than usual. In almost every case this rifle is inferior to the default version. But then, by total accident, you implement a vital base with a canyon chokepoint contested by cover 500m away from the defenders rather than 450m. Some founder discovers that a bunch of dudes, with this weak but long ranged sniper rifle, can slaughter enemy offense from further away than they can counterattack with non-purchase countersniping weapons.

And suddenly, from an otherwise balanced gun, you have gotten a PtW result. This situation is obviously not universally applicable, but my point is that this kind of stuff can't be predicted until it's too late. It is totally chance whether something ends up being PtW, and I don't want to see the game damaged by the products of that risk.