I just respecced fire after being arc/frost since release.
I am easily doing more damage than ever before, and I'm not really noticing the mana problems. Patchwerk obviously stretches it, but its still manageable, and i dont think "hey its more mana efficient" is a good enough reason to bring a frost mage to that fight.
one thing that IS hard to adjust to is ignite damage. I have died a couple times due to not managing aggro properly with the added damage. When you see that 3k ignite ticking away its easy for my eyes to light up as i go nuts trying to maximize damage, rather than just laying back and praying to not get aggro. but on non aggro fights we've been playing with ignite rotations, and its pretty fun to see those big numbers.
I'm surprised you say that - recently all of our mages respecced fire and haven't complained of mana issues much at all (Patchwerk being the exception). I think the major problem with Fire as Horde is that in nearly every fight, whoever gets lucky on ignites seriously might die. We had a mage pull aggro on Grobbulus about 30 seconds into the fight because of a huge ignite roll. The best part was - we knew he was about to die before Grobbulus even came to him. Pretty much every Gluth we do - whatever mage gets a big ignite roll, dies fast. Personally, that's major danger of being fire as Horde, otherwise just be prepared to use consumables a little more often than as Frost, and die a bit more.
Unless your whole raid is fire, and had the gear to support it (hint the entire NW set sucks for fire), then it's not worth it as horde. When I was fire, we had 2 fire mages, and 4 frost mages on raids. The mana efficency of scorch doesn't outdamage just pure frostbolt spamming until you have around 4-5 fire mages in your raid.