Originally Posted by RevGTO

Depending on the model of your car (esp. if it's a Formula or Z28) you can build your own "catback" for cheap. You definitely want to get rid of that horribly restrictive stock muffler. And longtubes are going to make for huge progress.

But the best overall performance investment you can make is a tuning program (EFILive or HPTuners). Frost is great, but you can learn to tune yourself and make adjustments many times over if you own a tuning program.