Uhh, what I meant is driving two wheels at a time, not a single tire.
I haven't seen proof that IRS is better either though, the Vettes do well with it, but past the 9s they usually go solid for several reasons, rules for saftey being one of them.
Now I see what you mean, IRS 1WD versus solid 1WD. Interesting read, I'll have to go over it again. However still, for the amount of power in a Z a heavier car runs the same times often, which still throws me although I'm assuming its level of grip due to the light weight car (lacking grip).