Yes and no, it sure seems like someone in endgame gear when level synced does have a small advantage over someone in leveling gear. The endgame gear has a wider selection of stats on it letting the character have a more balanced stat allocation that can significantly help. The gear is higher quality and that does show even if it's not a gigantic leap stat wise. When I heal a lowbie dungeon and have an ilvl 90+ tank I barely have to toss a heal ever, where with a at level tank I do have to pay attention to their health a significant time more.