I owned an '04 GTO with headers/exhaust/tune. It ran even with an '05/'06. Decent car but dam are they heavy! I'm still heavily involved in the LS1 forum. It is definitely not as easy as you think to run 12's in the GTO. MANY LS2 GTO's I have witnessed go mid to high 13's at 100-105. It really is all about driver in those cars. They are incredibly tough to launch and you cannot fit a very wide tire on back due to fender rub issues. I had my fenders rolled which allowed me wider tires. Good luck.

BTW here is a video of my '04 Auto vs an '04 Stick with the same mods...I am on the left and beat him by 1 tenth of a second!

http://media.putfile.com/GTOByron