I think you are misconstruing why Travis was more successful later in the game. It wasn't because he needs 20+ carries to warm up. It was more because he had worn down the defense with his pounding running style -- and he didn't wear down as much as the defense did. So when he hits the hole with first quarter force, he tended to get more yards.
I do not see this being a problem, as McGahee will do as well as Henry in terms of wearing down a defense. In that respect, the 1-2 never-ending punch of battering ram-style running, Willis and Henry could be fantastic. In theory of course.