Exactly :)

The problem with your trick is that you'll need the shell for it. And since you need mini Mario to get to world 7 and the 7-4 secret exit you keep mini Mario and therefore enter the castle as mini Mario. There is also at that point no really fast way to get the shell in your inventory. So you enter the castle as mini Mario and refill to fire Mario since this level is more or less an auto scroller. (Unless you have the capabilities to do TAS tricks of course ;) ) So this is why I don't think this trick is really useful.