Little... I don't have time to slap on a wheel set with 25s right now-- I can do it later tonight-- but eyeballing it, it's very snug up front, and maybe not possible, while the rear is roomier.

I've got a set of Michelin Pro Optimum 25s on another bike, which I think are pretty chubby as far as 25s go, so I'll give those a whirl later and report back.