20% is already a significant bonus to killing and questing XP, and rested technically provides a 50% bonus. But with ten Heirlooms in 10 slots (we'll leave out weapons, since those don't have the 10% XP bonus, as well as shirts and tabards, and rings and trinkets for now), you're looking at a 100% XP bonus even without Rest. The current average 80 probably spent about 14 days leveling up, so with an extra 100% bonus, you're looking at seven days /played, or very close to the current record. At that point, Blizzard might as well let us grant levels to each other.
Then there's the other point brought up: dressed up in Heirlooms, who cares about your gear? I'd argue that's not as big a deal -- when I'm leveling, all I need from my gear is enough DPS to get the job done, so I don't worry about min/maxing or going for the pretty stuff. But yes, it seems unlikely that Blizzard would ever outfit us in full Heirloom gear, especially with the XP bonus. Even if another couple of expansions take us up to level 100, just making the leveling faster doesn't seem like the best way to do it.