The C5 Auto was a lot less efficient at getting power to the ground than the F-Body. I have seen dyno tests from a speed shop and comparing 2001-2002 F-bodies to the same year C5's, the automatic F-Bods would put 15-20 more hp to the wheels. The difference for the 6 speed was a lot less, somewhere in the 5-10 hp range. That difference came from the IRS in the Vette being less efficient and the exhaust and intake being more restrictive in the Vette. I believe the GTO and the Corvette, 04 to 04 or 05 to 05 have the same rear ends and transmisions on 6 speeds, the Auto is different in the two cars. From the dyno results that I've seen comparing the 04' GTO to the Vette are almost identical numbers with the GTO having a slight edge (5rwhp) in automatic cars. For 05, the Vette has a redesigned tail end and some underbody redesign that allows a better flowing exhaust. I would guess that the two should be almost equal power wise.