The heirlooms would probably help in equalizing the mages' aggro so the mobs will bounce between them. But I would forsee some hard hitting low-level bosses being difficult to down without multiple mage deaths because at those levels you don't have the important abilities like Mirror Image and Time Warp.

Even with 5 sturdy boomkins, I find myself having to shift out of form and heal very often in low level dungeons, and I can only imagine that while mages do a bit more DPS than boomkins, they are a ton squishier and most bosses would be immune to slows or CC.