you can't really "add" power, per se, but you can move the power band around a bit.
first, let me ask: are you consistently running the bike above 7500 rmp? peak hp for the 2.5 is between 7 and 10 grand. so start with that technique if you don't do it already. the bike is VERY fun (even stock) if you keep the rpms up all the time.
next, get rid of your stock tyres if you haven't done it already. get a set of pirelli sport demons. they will cost you about $175 plus mount and balance. when the bike sticks to the road better, it will feel faster and more in control, which equals more fun!
after that, your only real option for any kind of hp increase is to get the muzzy full system exhaust (muzzy says 10-15%, which in our case is between 2 & 4 ponies) when you do that, you'll need to move the jets up to stage 3 and put a k&n air filter in the airbox. total, that mod will run you about $600 ($400- muzzy, $90- jet kit, $60- air cleaner) if you buy it all new. i bought a muzzy with about 5 hours on it off some dude on the starboyz.com board for $150.
other great "fun mods":
swap your stock fork oil with 15 wt repsol and replace the shock with a shock from an ex500 (run you six buck on ebay)- this mod will make your suspension stiffer and allow you to dial it in for your weight.
woodcraft rearsets: move your seating position back a little- now you can sit up or lay down, your choice. will also solve that nasty problem with the ground clearance where your stock footpegs drag on the ground if you get low in a turn.
double bubble windscreen- looks sweet and will help with that nasty wind gusty you get on the highway.
