That's the exact question you should ask yourself if you find yourself stuck in a spec you don't want to play at a mythic level.
"Am I willing to stop raiding Mythic to play this spec 24/7?"
If the answer is yes - then you should be playing that spec, consequences be damned. If the answer is no - then you should just suck it up for the prog and play what gets the bosses dead. The sooner it's on farm, the sooner you can start playing the spec you want. I'm assuming that most every single GM would be ok with off-spec play on farm content for the fun of it.