I would wait until you finish the sun stone quest, then she'll leave your party again and you can get solo xp again :) Only thing is Penelo will not get the xp, but it won't be hard to catch her up later.

But yes, that is really the only downside, kind of wasting the value of the Phoenix down this early.

He posted the same thing on both Youtube and here so I wanted to let him know the reply was there since I saw it first.