That site, the chart, and all the information surrounding it were quite informative and exactly the sort of knowledge I was looking for. Thank you! Not only did it have the answer I was looking for, but I stumbled across a clue about tire inflation and all the rock chips I've been extracting from my tires.

And you are correct, now that I look, there is not much extra clearance at the chain stays, for both the added width and the added tire radius. I would have perhaps 2mm of clearance, and that's making assumptions about the new tire's profile.

I think I'm going to skip this idea, and stick to the original size.