As a GT rider, I think he slightly underachieved. I think he had a quality for one more overall win, but stars didn't align.As a classics rider he probably underarchieved. As a gt rider he's an obvious overachiever. It levels each other out IMHO.
As a classic rider, I don't know, it's a tough call. 4xLiege, 5xFleche, Worlds + 6 medals, 2xSan Sebastian, Lombardia podiums, Amstel, Strade Bianche podiums, Ronde and MSR top 10's... It's a hell of a palmares. Had he been only a classic rider, like Bettini and Gilbert for example, I would say that he was been capable of even more , but we must take into consideration that he rode everything and everywhere and had other priorities as well, so he couldn't time his form perfectly for those races every time.