From Gustav videos, we can see the Z06 is clearly the fastest of all . M5 Kellners is just a tinny slower than the 997TT but this same M5 Kellners when raced against the Z06 it stayed far behind. The Ferrari also seemed to gain more ground to the M5 K than 997TT did. I wasn't really expecting for this result, offcourse you can easily tune the 997TT and smoke all the other's, but it should be faster from stock. Anyway I believe if they have started from 0kph 997TT would have stand up better against the competition.
I can only say the Gallardo must have a blown MAF or any other engine bug, that's not possible to be so slow against the heavier M6 using same engine.

J.Seven