Vanguard with a Reegar at 200% is pretty nuts, specially with that incendiary.
Yes, the Reegar is overpowered, and it synergizes extremely well with bugged IA stacking.
One thing to consider with respect to SP is that the guns (and actually many powers) were buffed with the last patch to make them the same as in MP, but enemy HP was not buffed. If you played the game on Insanity at release, the end was a bit more challenging since weapon damage was a lot lower, and there weren't DLC guns.
Hopefully MEA strikes the right balance between powers and weapons. Individual cooldowns is not going to help though. And making powers so effective you don't need guns isn't either.