I boosted to 45 with a my reasonably geared 80 mage then switched from 45-67 to DK then quested the rest of the way. Unless your 80 mage is well geared I would think the limit would be around level 60 or so. Stuff just hits too hard to aoe effectively after that and single target killing slows the XP way down. With plate you can still aoe kill effectively enough to equal quest xp - if you are boosting healers you can probably boost the whole way wearing plate (although questing is still probably faster).