The one problem I can see with the high ilvl synch ceiling is that every now and then you're going to have a party with a fresh lvl50 tank and ilvl-synched dpses. Which is going to make aggro maintenance a chore for both parts (tank desperately trying to keep the mobs and dps tempering their frustration over having to wait with spells/hits when they hit orange markers on all mobs). Or a new healer desperately trying to keep up with the size of the pulls the 'experienced' players think they should be able to handle.

If anything, have the ilvl-synch relative to the party members. Say "synch to 10 ilvls above the average ilvl of the lowest party member at time of entrance" or something. That would fix issues of low-geared tank + high-geared dps, and would give first-runners a taste of what the dungeon was closer to the beginning.
Sure, people could troll that and put on fake gear to just about hit the ilvl requirement, and then change once inside the dungeon, but then again people can troll almost anything -- since you can change gear even to below ilvl requirement once inside, you can just switch to unglamoured event gear and go trololol through it all.