The producers and the BBC are going to want people to approach series five saying "There's a new Doctor, I wonder what he'll be like," not "There's a new Doctor, except he's not really new because we saw him in action fifteen months ago." The best performance in the world wouldn't be worth jumping the gun like that. Also, if they wanted to trick people into thinking he's not a future Doctor and then pull a plot twist, the promotional material now would be saying he was an impostor, rather than playing it cagily as it has.
Well, you've outlined the conventional thinking. But things change and people push the envelop to keep things interesting. I'm not saying it will happen. But, I don't think it would be a bad thing if it did. If the new guy did a bang up job, people would be more psyched than ever to see him in action. Basically, you're changing their mode of anticipation. You're chaning it from, curiosity about the unknown to excitement about a new known factor. I think either works.
Mr Awe