The INT boost for XP is pretty small, I've found. I went up 3 points from clothing at one point, and I tested the increase. It was like 25-30 XP more for a quest that gives you 325 or something close to that. If 3 points is less than 10% - I can't see it being a problem, though I don't know if the increase is exponential, as it may well be.
It doesn't seem to affect the monster XP - at least not so far.
I'm playing with default XP rate - and I'm only level 17 after almost 30 hours. Considering how leveling seems to slow down as you gain levels, and they have perks and mobs for well over level 50 characters, I'm quite happy with the rate of leveling so far. That would make me level 50, at most, after around 90 hours.
That said, I do a hell of a lot more free-roam exploration than quest solving, so that could be a significant factor.
I expect to be "full" of my first game after around 100 hours. At that point, I don't expect to be maxed out - or even close to it. To me, that's fine.
I could never play these games for hundreds of hours in a row.