Ok, you've just opened a can of worms...
1) ANY aftermarket exhaust will give a little bit of performance gain.
2) 95% of those gains come from just ditching the stock exhaust. Which means your argument is only valid for 5% of the gains (we are talking maybe 3hp? if that?) - so 5% of 3 HP. You are looking for 1/5th of a hp (again, this is not fully scientific, just trying to prove a point.
THEN, you add on the complexities of whether you do a 900rr mid-pipe conversion, lose a bit of mid-range, but gain on the top? AND/OR adding in a Power Commander, with at least 3 dozen different, separately dynoed maps made at different elevations, on different machines, with different bikes, and by different operators.
My vote? Pick the aftermarket exhaust you like for SOUND first, THEN looks. And forget about the rest. Well, actually, no, I would recommend a Power Commander, because that can really smooth out the throttle.
And if you want to get really picky, get the pipes you like, then get your bike dynoe'd (will require a PCIII). That's the best way to get the most out of YOUR bike.
2009 Aprilia Tuono - Ginger
2001 XR650R BRP (Big Red Pig)
2006 Honda 599 - Ex wrecked it :-D
2007 Honda CB900F (sold)
2006 Honda VTX 1300C (sold)