Well if your only talking about slip-ons, that won't give you much HP, maybe 2 or 3 tops. If your talking about a complete system (including header), yes that will give your some HP. I got 12 hp at the rear wheel with my Gixxer when I installed my Hindle full system with 4 to 1 header and K&N filter and some remapping of my ECU. Now, my bike went down a couple of weeks ago (yes, with me on it) and it ripped off my Hindle exhaust so I had to reinstall my stock one and I can really tell that I lost power but I ordered a D&D full system and should be installing it as soon as I have a couple of hours to do it.
Most popular mods on bikes: exhaust, K&N, changing the rear sproket for more acceleration, stearing dampers and then most guys go for look more than performance. It's very rare you see someone change anything in an engine on a sport bike.