Good discussion, I am a 4th gen Trans Am veteran before my 69 and I never saw too many people go away from bolting on parts to the stock style Torque arm suspension. Like others said, you can do a nice setup with any of those companies like BMR, Spohn, UMI etc and the prices of the parts are relatively cheap. One of my good friends does a lot of auto crossing his 4th gen and he says with the right bolt on parts he can spank C5's and C6's. He stresses brakes, tires and shocks are most important.
I always have my eye out for clean Fox body Mustangs and 91-92 Trans Am GTA's. Heck I almost bought a 91 GTA recently but found something else
I love the cars from the 90's even the RX7's, Supras etc it was a fun time.