I had 650 lb/in coilover springs in the rear of the Cobra. (450lb/in coilovers up front). (1600 lb/in wheel rate)

It still needed work out back.

The bigest problem with the IRS is the bushings. Under load, they compress and eventually one side looses traction and the bushing unloads. (causing a wheel hop). It returns to a neutral position and regains traction, but the shift causes the other side to loose traction and hop as well. The only guys I've know to truly 'eliminate' wheelhop have replaced their bushing with delrin (it's a hard nylon like substance). That doesn't even eliminate the wheelhop. It's just so hard and compresses so little that the frequency of the wheel hop becomes so high that the wheel essentially hops 0 distance. Because the travel is so near 0 you don't do any damage to stuff.