I am the wrong person to ask. I have owned an XO1 frame for about two weeks. I have never seen one on the hoof, much less ridden one.
But, I am huge fan of narrow tires on this type of bike. I would think that 1.5 would be plenty wide for most any sort of on-road use. Heck, I would even suggest going smaller than that, even for touring. Maybe especially for touring. Sure would make for a long tour to have to be staring down at the fat marshmellow under you the whole time.
jim
__________________
Cross Check Nexus7, IRO Mark V, Trek 620 Nexus7, Karate Monkey half fat, IRO Model 19 fixed, Amp Research B3, Surly 1x1 half fat fixed, and more...
--------------------------
SB forever