I just got back from the last open road race with my 05 GTO. The C5 Vettes and 05 GTO run virtually wheel to wheel in the 1/2 and mile shootouts. Coming in in the different speed classes is purely driver dependent. A good driver can take either car and kick your a$$ on the open road course.
The C6 Vettes are faster, a factor of weight, but not that much faster.
Automatics are easier to launch and actually faster 0-60. If you like rowing through the gears, fine, get a 6 spd. I think the auto is actually a better choice for some applications.
I have owned 7 Corvettes, this is my first GTO. In my opinion it is a better all around car, much nicer to travel in.
No car is perfect, including the GTO, but it is SERIOUS value for the money.