How much power can the autos take

Apparently a decent amount.

From this article -

http://www.alternativeauto.com/waterbox/wb_archives/supercharging-05-gt.html#methanol

"So far the 5-speed automatic in the 05 GT is holding up very well. We pounded our 05 GT for over 8000 miles this past summer. The Vortech has been on for well over half of that. I’d have to say that in the short term the 5R55S automatic has more then exceeded the life expectancy I predicted it would have behind what’s appearing to be about 525+ HP at the flywheel."