I used to belong to the GTO forum (got banned eventually). I had a 2005 GTO from April 2005 - April 2008 when I replaced it with an 08 S2000 which in turn got replaced with the SRT6 in April of 2011.

Some of the guys on the GTO forum are decent guys / true car nuts. But as the cars began to hit the used market at discount prices I began to see a lot of younger guys who didn't know much about the car showing up.

Anyway, having owned both cars, I can say that stock for stock they are pretty much dead even in a straight line. At 400/400 and 3,800 lbs the GTO was advertised as a 4.8 second 0-60 car. At 350/330 (real factory ratings) and 3,200 lbs the SRT6 is likewise a nominal 4.8 second 0-60 car. Where the differences lie is that with forced induction, the SRT6 can easily and cheaply be upgraded with a pulley and tune to something like the 400/400 GTO specs at 600 lbs less weight. The GTO on the other hand will never get 600 lbs lighter.

Also, the AMG 5 spd automatic in the SRT6 is really well mated to the engine. The Tremec T56 in my GTO was serviceable but far from the precise instrument that the 6 spd Aisin gearbox in the S2000 was. Given 2 equal drivers and 2 stock cars, I'd give the edge to the SRT6 simply because the AMG gearbox will be much more consistent.