I would suggest sprockets as the first money spent. If your heavy like over 230lbs, I would run a16/43 or just a 46 in the rear. Great gearing for both street, drags and road course(could vary for some track configs).
Do the free mods as well. Weight removal, airbox mod. If you think you need to do a tuneup on the bike just for your sake of knowing what maintenance has been done, change to Full Synthetic oil such as Amsoil or less expensive Rotella, change to Iridium plugs gapped a little more than stock and keep the chain well lubed. Don't bother with all short stacks in the airbox unless you are doing engine work. I as well as 100s of Busa owners have proven that the stock config will yield best results on a stock setup.
Once you have mastered what you have done as suggested, then you can start going forward in spending every dime you make on things such as exhaust, BMC Race filter, ECU editor flash module, dyno tune and anything else you can throw at it.
Hint: The stock clutch torque limiter is hard to modulate for drag racing but works really good for road racing when you downshift at ridiculous speeds and rpms....almost works like a slipper clutch and when blipping the throttle with it works great. Otherwise for dragging get the clutch mod.