Earned xp does begin to taper off, because you earn it on a miss and high-level PCs miss their rolls less often. Doubling it seems harsh, but you could cap xp per session. You could also play around with less end-of-session xp.
There was talk of a high-level supplement, to me that makes sense. At level 10, adventurers are a Big Deal and probably are handling statesmen-level stiff and not dungeon delving. That's the usual progression, anywyay.