Didn't want to go FI ($$$) so I added the ZZP ECU tune. Nice gains in throttle response and a rock-steady idle. HP gains (with an aftermarket CAI) -- Maybe 5?
Didn't want to go aftermarket coilovers (again, $$$) so I added DDMWorks' handling kit. Nice smooth ride with more compliance over bumps, razor-sharp turn-ins, better cornering. The lower stance looks great too.
Of course, the DDM kit was supplemented by a heavy-duty Twist Dynamics sway bar, which keeps the front wheels planted and helps keep the Sling board-flat through turns. Of course, the Bridgestone Potenzas also contribute to the handling.
Overall, these improvements are nibbling at the margins of performance but have enhanced both performance and driveability. $$$ well spent IMO.