I, personally, haven't seen anyone swapping out a mage or shaman for hunter MS. and IMO, it is not needed. Range dps' problem is Not not enough damage. It is mostly that people run away from you. No healing is enough to heal through if your target just stand there and let you free cast on him. Hunter seems to be out of sync in both groups (attack, burst rotation). So I would suggest stick with 4 shamans and 4 mages.

Disc priest is good. 2nd to none as healer. But if you plan to play all 5 yourself in arena (as you controlling the priest where shaman follow), that might be difficult. Not impossible, but can be frustrating.