i have the exact same 5 box team at 80 and clearing most heroics with ease and Im lvling a hunter to replace the mage.

heres why Im droping the mage...

my boomkin and mage are around equel on dmg
boomkin offers 13% more magic dmg + haste + better buffs than a mage + in combat rezz + innervate + extra emergency healer
mage must be frost(worst dps spec) or youl drink every few pulls when doing heroics
best thing the mage brings is replensihment which the hunter gives anyway.
i use a portal maybe once a day if i play all day.
i drink maybe once an instance so free drinks arnt that big a deal
boomkin boe epics are seriosly cheap compared to cloth on AH
boomkin can take a few hits, mages just cant.

few tactics i couldnt use without a boomkin
tank dies - drop earth ele - in combat rez tank.
healer dies - heal with sham - incombat rez priest(innervate priest)
i have a great macro that takes over healing with boomkin (when u heal moonkin for drops, on non moonkin action bar in my spam dps spot is a spam heal macro so an almost seemless switch from nuking to healing with 1 button press)


And why Im subbing a hunter
still gives replenishment - non stop chain killing in heroics
gives a melle pet - this sounds like a small bonus but it has a few huge benefits. 1 i love is as the pet takes a bit of dmg it'll ping prayer of mending about, it'll eat those randon non-tank hits/fears etc.
takes all of the melle dps gear that goes to waste
i cant be 100% on this but Im pretty certain will well out dps the frost mage when in even gear

oh and if you do get a hunter up. check the wiki out is has a perfect dps macro up there Im just loving.