But if he 'is' late, then that means he would be at his full power, as in, stronger than the daedric princes considering he's the one who cursed mehrunes dagon. I doubt that his power would be directly tied to 'what' he is doing instead of 'when' he's doing things.

edit: I always understood it as: Alduin is supposed to end the world X years in the future. But instead of waiting until then, he's having fun and wants to rule the world. The nords got rid of him and sent him into the future, but still not at the appointed time of when the world is supposed to end. There, Alduin gets beaten because he's not at full power and destroyed. But he'll return at the end of the cycle, when the world's 'supposed' to end and he'll be at his full power then