An IRS would be awesome, but you can make a stick axle perform very well (very close to IRS levels). The main issue with our rear suspension is the very short upper control arm, it's far too short for proper geometry, which is one of the main causes for our wheel hop issues. A torque arm setup would eliminate this issue, griggs sells one. Speaking of griggs, yes their kits are very expensive, but their race front kit claims to remove over 250lbs from the nose! And is an SLA setup, which is better than our MacPherson strut setup, which in turn frees up enough room to run 315 tires up front!
Ironically, the 5th gen F-bodys used an SLA front setup, so they went a little backwards up front while getting more advanced in the back. Food for thought though, the M3 uses a MacPherson strut assembly very similar to the mustangs for their front suspension, and that's on a 3700lbs car.