They could just have the gear sync down as well, reducing its effective stats. They do this in XI and in other games with a similar system as well.
When the topic of level sync comes up, almost without exception, the reason given is "I want to be able to play with my friends and/or family members who may have started later and are too far below my level". That is almost always the reason given and seems to be in line with what Yoshi-P wants to support as well.
However, the idea of it being (ab)used as yet another way to PL could lead to issues similar to the current PL'ing mess, which, based on a translation in another thread, certainly seems to not be what Yoshi-P had in mind. He still has that 2-3 month to 50 time frame in mind.
That being said, since the purpose is to be able to help lower level friends and family members get through content and catch up, then I think xp should be reduced down to, say, 25% for the player sync'ing down, once they're more than 10 levels or so below their own actual level.
Why this reduction? Well for the following reasons:
1. Allows the friend/family member to catch up that much more quickly. If the higher level player is gaining xp at a normal rate, then that gap is essentially never going to close until they hit level cap.
2. Mitigates the fall-out of FFXI's implementation, where high level xp'ing parties became extremely difficult to find or assemble 'cause everyone was sync'ing in Qufim Island or some other lower level area.
3. Prevents sync'ing from becoming an "all-purpose" xp'ing option in all situations. Rather, it becomes very specific to certain situations... ie. helping lower players out.
Even if it's not that specific setup.. as long as it's implemented in a way where it doesn't all but kill off high level xp parties as it did in XI (Abyssea notwithstanding... that's a different trainwreck altogether).. I think it would be great.
Now... some might scoff at this idea. However, assuming the numerous claims that it's "purely to be able to play with lower level friends and family" are genuine, and not just feigned altruism, I have a feeling many who'd be against it are those who want to power level themselves, and have very little, if any, interest in "helping anyone else".
That might seem cynical. However, the cynicism is warranted I think.
In FFXI, every hour of every day that I was logged into that game (and certainly even when I wasn't) there were people level sync'd down in Qufim Island and elsewhere, xp'ing. People I knew or knew of constantly xp'd in level sync parties and would never join higher level ones anymore. I have a very difficult time believing every single one of them was doing it "to help a friend or family member" or, were doing it as an act of altriuism, "to help some poor lowbie out".
In fact, to punctuate and underscore the issue XI's implementation brought... a number of them were sync'ing simply because that's the only way they could get xp at all. High level zones were all but dead. It became so "ingrained" in people's minds that "xp'ing = level sync" that you couldn't get them off that mindset.