If you are going to throw your car around on a track, the absolute 1st thing to work on is the oiling system which allows the engine to ****/burn oil during hard braking.
The first time you experience this you will be terrified because you will have thought you blew your engine up! (white smoke out exhaust and knocking sound from oil induced detonation)
Next mod is BRAKES, BRAKES, BRAKES, ...
Min mod is Porsche brake deflectors.
Brake upgrades... there are so many and I haven't don't mine yet. Big decision though which system to go with. My selection criteria are pad and rotor costs since these are consumables.
Regarding HP:
I don't think minor HP improvements are worth the effort - major ones are.
I don't like nitrous because the bottle is always empty when you need it.
I like supercharging and NA modifieds.
If I wanted 600 rwhp, I would stick with NA.
If I wanted a lot more, then supercharing is the way to go.
However, on a road course, for me, HP is not the thrill, cornering and braking are. Drag racing, however, is HP HP HP.
...but the car is so superior totally stock! ...and I really get a kick out of that!