I've never rushed to finish the Main Quest - it's kind of a back burner thing in my games. However, I have a friend who played a pure mage (all perks in Destruction, all leveling increases spent on Magicka) and she defeated Alduin at level 19. That seems pretty quick to me (total magicka only at 290) but is it? What's the lowest level at which you've finished the MQ?