I did a search but found nothing on a compilation of aftermarket exhaust hp increases to identify which exhaust makes the most power. I would imagine most full exhausts should be similar in hp increases, and most slip on exhaust should be similar.
How it sounds is a personal thing, I am not a fan of real loud exhaust but others are obviously.
Does such a compilation exist?
From what I've seen over the years, there are lots of variables to consider, dyno, tuner, elavation, weather, individual bike, blah, blah.
Most aftermarket exhausts make similiar numbers, but Brocks, Akropovic, and Ti-Force, as well as the side exit, and the old HMF Big Birds, seemed to make a little more than the competition.
You could also argue what's best for the street(useable power throughout the rpm range) vs a drag pipe(making more power higher up).
But it really does boil down to just whatever you like best. In most cases(with full systems) you're not really going to feel alot of difference between pipes.
Maybe the placebo effect if you switch to something louder.
For hard facts, this is more of a PowerHouse question. What have they typically seen for non turbo exhausts(when tuned).