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Removing the 15% cap would make armor super OP. A pulse laser would do 5 damage to an onslaught. Small ships would be totally unable to do any non-trivial to high armor ships. I could see an argument for a reduction in the armor cap to ~10/7%, but you have to be super careful messing with that, especially when you consider how the balance works with skills. Full armor skills are super strong.
The strong part of armor is that you can absorb damage without increasing your own flux, which lets you win the flux war easily. Selectively block high damage shots with shields and kinetic/low damage with armor is very strong. The whole point of armor is selectively avoiding damage on shields to win individual fights and gain an advantage in the overall fleet battle, and extra armor definitely helps do that. Battles only take a finite amount of time, and you're trying to maximize your performance in that time, not over an arbitrarily long time. If you use your armor to kill the first 30% of the enemy fleet, then you can often steamroll the rest of the fight because you have a numerical advantage.
I think HE torpedos that wreck armor are balanced by their susceptibility to PD and their limited ammo. I also think they need to be very effective against armor because other weapons are completely shut down by high armor. I don't think that is a problem. Armor is very effective against a large portion of weapons, and you can choose to use your shields to stop high damage HE stuff.
I would also argue that a salvo of sabots does negate shields when it overloads you.
The maneuverability penalty on heavy armor is too much though, I agree with that. The extra armor is definitely never bad, it basically just increases the amount of time you get to spend taking 15% damage from most weapons, which is significant.
TBH, I think the armor and flux mechanics are the most well designed part of the game. Maybe some numbers can get tweaked here or there, but overall, the balance is very close IMO.
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Ok 4k might be a little too much, but isn't that what the onslaught supposed to be? With its low flux and obvious weakness to flanking, isn't it supposed to have very high armor on the front to encourage smaller ships to flank?
Especially ships like onslaught who spend 80% of the time fighting at max flux due to its *** flux stats.
While on paper it sounds op, remember that armor, unlike shields, it doesn't regenerate. Damage piles up pretty quickly (especially by anti armor weapons) So 2k armor gets "consumed" pretty fast in battles. And we all know once armor reaches 0, it is a quick death in most late game battles.
While speed is part of defense, in the current meta, fast ships die way less than armored ships. Because fast ships do their jobs properly whereas ships that are supposed to eat shots, just die without doing much since their armor doesn't protect them from the various anti armor shenanigans that the game has.
This is why players in general prefer high/mid tech ships more than low tech ships. The latter have stats that allow them to do what they are supposed to do.