I think the good thing is that we will find out. I am thinking that if the Z/28 doesn't have any less weight, then the nod will have to go to the GT500 all around. We just won't know though until it is out and real tests can be done.
Either way, I wouldn't really view either car as a looser and would want both in my stable
It looks like they will be able to obtain 650rwhp / 650rwtq pretty easy on the Z/28 since that is what pulley changes and a different cam has gotten at a well known tuner in the area on a CTS-V..
Crowley