A test a while back in one of the Canadian magazines (forgot whether it was Cycle Canada or Inside Motorcycles) found that, with a Suzuki GSXR1000, ALL aftermarket exhaust systems gave within a couple of horsepower of the same power output. All of those were 4-2-1 systems, but for all those tapered header pipes, hydroformed Serpent header pipes (Micron), cross-over tubes (Yosh and one or two others) ... they basically didn't make a darn bit of difference. Some of them were louder than others, and some of them had some mid-range issues, but basically there was no real difference between them. I recall that Akrapovic was among the better ones in not affecting the midrange and not being grossly loud, but a cheap (locally built) Hindle was essentially 90% as good for half the money.
The only thing I can recommend to *not* buy is a D&D. Poor muffling (too loud), they sound raspy and nasty, and the experience that I had on an FZR400 D&D exhaust is that they are not tuned properly to the engine. The home-made, designed by myself, hand-welded 4-into-2-into-1 that I built myself makes the same power, is quieter, and doesn't have a horrible hole in the midrange.