Vortex rearsets - $300 gives you the ability to completely change the ergonmics of your bike without having weird shifter angles
lithium battery - $300 for weight savings
ABS/Emissions delete $700? - 10+ lbs and frees up tons of room under the tank
Lightweight wheels - ?$?$?$?$ 10ish lbs of rotating mass completely changing the characteristics of the bike making it ride and handle much lighter than it already does
44/45 tooth rear sprocket - $45 really wakes the bike up regardless of your riding style without sacrificing any freeway behavior (or $200 to go 520 like I did)
If I was going to do my bike all over again, this + the carbon headlight is where Id start (not including full exhaust, stacks, filter, tune) I think between those listed above and the ehxuast you'd also cut 65+ lbs of weight both sprung and unsprung weight.