As a guy who does a fair amount of MNKing (when I'm not having to tank/heal, sigh), I don't think I've ever run out of TP on a standard dungeon boss; none of them live long enough. There's a couple Crystal Tower boss fights where I can legitimately run out of TP if it's a bad DPS group and the circumstances are right (Hein where I never get frog'd/frozen, Five-headed dragon without being the sneeze target, Cerberus when I'm on the outside), but that's about it outside of Coil.