Just saw the Super Snake on AOTS A few days back looks legit. Unfortunately PD probably won't add something that extreme this late. I have to imagine that with the PSN deal PD is probably trying to mess with the servers or something for the rebuilt PSN. Keep up hope!
Possible? Yes. It's possible that they could add a thousand more Premium cars to GT5. Is it probable? No. There's all sorts of cars they could add to the game, but it's not very likely that they will add very many of them, and it's entirely possible that they might not even add any at all. I would imagine that they would make some sort of car DLC available at some point, but it's anybody's guess as to what cars those might be. It could be the Super Snake, Audi R18, or the Batmobile for all we know.