How to spend your money...
One thing not mentioned is maintenance...super cheap, and if it hasn't been done regularly, you'll see a marked improvement.
- Fork oil should be less than 1 yr old. Change it if you haven't already
- Plugs and air filter would probably yield as much HP as a $500 slip on, if they are not in great shape (didn't notice how many miles on your bike)
- Brake and clutch (if hydraulic) fluids should be changed at least yearly.
- I like to not let me front pads get too thin. Thin pads get hotter and fade sooner. I'm not entirely sure why, but I think it's because the brake pad material is not a good heat conductor and therefore insulates the backing plate while also shedding heat. In any event, when pads get a little thin, you'll feel it at the end of a long straight with a low-speed turn. Upgrade your pads, and have a spare set with you always. Or change your stock set now while they are still good, and keep the stocks as your backup.
- Get frame sliders if you haven't already
Maintenance is not glamorous, but it's cheap, easy and effective.
Dan B