Change your sprocket but you might lose your torque a bit.
It depends which way you are going with the sprocket. If you increase the rear you will increase the torques and decrease top speed and vice versa.
I wouldnt recommend a full exhaust, your low and mid range rmps will become sluggish. If you are an average street rider then I would not go with that system.
How hard are you hitting the higher rmps?
Most people just get an after market exhaust and a powercommander. This will optimize the bikes performance. If you want to do engine mods than I would say just get 1k. It makes way more sense to upgrade to 1liter than wasting money on a 600.