I think you will get even more varied replies than normal because you're kinda chasing a unicorn here. You can make decent gains and loose the most weight with a single outlet exhaust, but its gonna be loud as hell. You will also loose a decent amount of weight with a good dual outlet, just not as much and most make some power over stock but they will be more quiet while still sounding aggressive. Basically it is hard to be the lightest most powerful exhaust without being pretty dang loud. Just the nature of the beast...

 

in that case, T1R 70mm.

 

JIC titanium single exhaust. Weighs 12lbs. Hate the sound.