It is speculation like this that makes me want to wait until Sep/Oct to buy a GTO. I would hate to buy a car that will get a 50 hp boost during its second year of production. I am sure dealers will ask for an arm and a leg for this 400 hp option---kinda like what they are doing now for the LS1 V8.
If it turns out this is nothing more than a rumor and the 2005 models still retain the 350 hp LS1 then I am sure I will get a better deal at the end of the year when the 05's come out rather than pay these ridiculous dealer mark ups for the current goat.
50 HP isn't all that hard to come by on the LS1 and, while you can always do the same mods to the LS2, early adopters will pay dearly for the first couple of years for anything other than bolt-ons. Fbod, Vette, and Holden owners have done a lot of free R&D for us.