Slingshot drag racing/roll racing traction strategies are very different than road racing/autocross traction strategies.
Power over 300hp won't help you on most road-course tracks. In autocross a NA Slingshot is basically just as fast as a turbo model. It is very hard to get traction out of corners in 3rd gear, even with the right tire setup. 2nd gear is a total spin fest, so in most cases, you just run 3rd gear pretty low in the RPM range and don't waste time downshifting.
I am hoping to tune mine in at just short of 400hp at the wheel. I know that extra 100hp won't help me in autocross and my normal track days, but it should make 1/2 mile roll racing a lot more interesting.
The high end turbo kits are cool! But they are more of a showpiece than a magical kit to make your Slingshot way faster than the other kits. Want to be the fastest? Better start with a quad kit.