There were mages in my guild who went in to T5 content with just 6kish buffed hp without lock's imp, and they still managed to survive then. I have no clue about the statement I read just above about all mages having IB now (quit the game like 6 months ago) but back then when mages were speccing fire or arcane, they managed to survive fine without any problem. All dpsers not only compete for top honors in the dps list (just for the sake of fun), but we too compare and make fun of those who die too frequently (including trash) and eventually that could take a toll on their call-up into a raid.

Most damages soaked in T5 content are avoidable, and for the unavoidable ones their relatively low hp should be able to cover it, if not just blame poor healing should it be the latter sort of damage soaked. T6 on the other hand is another ball game, but if you hit that stage, you should already be familiar with the mechanics of the raid and your gear should be rather sufficient.