chain is right - you will not add value to the bike by 'upgrading' parts.
But if you must - a full exhaust and good tune will yield 7-15 horsepower.
IMO, the best performance upgrade you can make is re-springing and re-valving of front forks, and a racing shock. Better handling is far more rewarding than more power.
...but I'm guessing you want more power anyway. honestly, it's cheaper and easier to CHANGE your power than add more. go -1/+2 with your gearing, and the bike will accelerate significantly faster.
also remove all unnecessary weight (body-weight included). the most dramatic changes will be felt with lighter rims, and other un-sprung components.
stainless steel brake lines and better brake pads (vesrah) are also a great upgrade.
aftermarket rearsets will make the riding position more aggressive, and also likely reduce weight. Vortex are my favorite, and look good.