Well, I think this goes to show the importance of aerodynamics. The 'vette had a better power/weight ratio and probably a better power/drag ratio as well. A 2,915 lb 'vette on wide tires is going to achieve some serious mechanical grip.
In addition to being cheaper, the ACR seems to have more time left on the table. Put those Ferodo pads and brake ducts from the Clubsport on the ACR, and I'd bet that braking disparity would disappear. Toss exhaust on the ACR, and it'll only get worse. In the end, 'vettes still have a huge advantage in the power department because of the availability of tuning software and cams, though.
I'm curious as to how C&D achieved a faster lap time with a base ZR1 than the Clubsport (which they had as only being 1.5 seconds quicker than a base Z06 - tires alone should have achieved that!). The ZR1's tires are probably better than PSCs under normal conditions, but I doubt they are better after multiple laps. The only thing I can think of is that Katech doesn't advertise with C&D.