they are the same except for menus / language.
think about it, if they made changes to the american version (buffing certain characters, or making others weaker) it would be game breaking to play on wifi against people with the japanese version.
also remember how people were showing proof that Sears had sold it early on accident. for sears to have had it already, it must have been shipped a few days before that. Theres no way you can make a change to a game and have it released that soon.
And that brawl tourney that was hosted by nintendo was using the American version. That may have been like a demo disc, or it could have also been the final copy. if it was the final copy that means theres no way there was any changes made AFTER they had already made some.