Pobst just drove both on the same track same day. Almost even, but the Z/28 was a couple tenths faster. Considering that those brakes are a $20K option on Porsche's, I still say this is a bargain for what it can do.
The gtr platform came out in 08. 6 years is an eternity in the car world. The new one will be out soon and I can see it dipping into the 10's like the 911.