Even a "serious autocross" car will see next to zero improvement with a coil over suspension conversion.
Consider this basic rule in life: If you are going to replace something, it should meet or exceed the standards of the original components.
The coil over suspensions have ONE advantage over a well built torsion bar suspension : Exhaust clearance. That is it. Name any other advantage that you have heard or read from an owner or the manufacturer and I can argue against it.
1) Better road feel: Better than what? A 100,000 mile original car? Sure. Better than a stock manual steering? Well, many of the steering racks used in these coil over setups are 16 to 1, that is good. The same ratio can be found in aftermarket steering boxes. Net gain? ZERO.
2) Better handling: Uhhh, better than a worn out car? I agree. Better than an upgraded stock setup? That is highly debatable. Net gain? ZERO.
3) Quick ride height changes: Quicker than a 3/4" socket and ratchet on a torsion bar car? Net gain? ZERO.
4) Increased exhaust clearance: Okay, the coil over setup wins BUT in most cases, TTI or Dougs makes headers that fit around the torsion bars, trans linkage, etc. Net gain? Actually, a Net LOSS. The coil over suspensions with brakes will cost over $5000. A proper rebuild with new torsion bars, brakes, shocks and sway bar can be done for half of that.