The CTS-V is a much more refined vehicle than the GT500.
It's suspensions is much better and can handle nearly as well all while weighing almost 600 more lbs.
The CTS-V is capable of 0-60's in the high high 3's completely stock.
Even with more power and lighter weight than previous years, the GT500 struggles to put up the performance (acceleration) numbers it really should. The GT500's downfall is traction, which is due to not only an inferior suspension set-up, but because of the tire width.
Remember, the CTS-V also has 285 wide tires (Michelin Pilot Sports) but has zero issues blastig out of the hole and has a significant amount more torque than the GT500. That is suspension superiority right there.
Notice the trap speeds. The GT500 traps 2mph higher, which is significant. If the CTS-V were a 6SP Manual, it would have trapped higher than it did.
Having a 2011 CTS-V (4-door), an M5 and a Mustang in the garage, it is real easy to compare between them all. The CTS-V is so much more of a car than the GT500 in every way, but the price also reflects that.
The GT500 is, however, the greatest performance bargain ever made. (And it's still overpriced, IMO) You can't get a better performing car for the money, and it's aftermarket power potential is extreme for little $$.