at the end of the novels?
TWFX was already the pinnacle of YGC's art and XMCX, although was unable to overcome it at the Duel, he had a close fight. By the end of the novels, he improved a lot so if YGC didn't, XMCX is likely to be able to best YGC.
So the question is...did YGC "max out" by the Duel?