The word "Perfect" has never been accurately used to describe any feature in an MMO because they grow and evolve all of the time.
With testing and patience, it could be made to be very close, but if you think that the current system is "Perfect" the way it is, you're very mistaken. Even now, with level syncing and item level reduction, people slaughter old dungeons. A brand new level 58 with quest or vendor given gear (you know- the normal gear that's available to a brand new level 58) vs a level synced 58 with reduced level 70 raid gear is going to notice a huge difference. The level synced guy is going to absolutely dominate the DPS chart. The system now is far from "Perfect".
I don't understand why it would be better to the game to keep this current system when it's clearly not perfect. I mean, if your concern is that the system I proposed would have to be perfect, aren't you concerned about the current system that's in place? Because the level syncing isn't perfect either. It's unbalanced and badly. Doesn't that bother you?