You can play frost for all ToT fights on normal and heroic. To the poster above me, Pyromelter, it's those little hidden lines that bug non-fire mages:
You don't need the word most there. All. All is the correct word, because the assumption that you have to be the highest dps spec when all of your spec options are viable in relation to the raid, especially at this point in the tier when gear has completely negated any marginal differences for DPS checks... I think the point is made there....but you can certainly still play frost for most ToT fights.
Nobody is trying to argue that frost outperforms fire. What frost does though is outperform other classes. And on its own, frost performs just fine. You shouldn't feel compelled to change. If you feel you're underperforming as frost, then you're probably going to underperform as fire or arcane as well.
Most of your DPS issues are going to be with timing of things and snapshotting which are present among all 3 specs.