[VADER SS L99]

I would say cheapest, other than nitrous, would be the off road catless mid pipes with a cai and tune but nothing beats LT headers, cai and tune combo for the best bolt on power gains. Track gains would be a properly selected high quality torque converter for the L99 cars hands down single best bolt on in terms of track gains vs cost, other than FI and Nitrous of course. Catback exhaust would probably be the worst bang for your buck mod as far as performance goes.