the 245 should work with a donut too. it's only a 1.7% difference and the donut is supposed to be just temporary. As for the 275s, my 255s are pretty close to rubbing struts on the inside, I would expect you would need spacers or even yet, wider rims with a greater offset for clearance.
The guy that did it used stock Vulcanis rims, so maybe the offset is different. If I were you I would go with the 255's. They are tested, and work well. Plus with it being winter, 275's will be really floaty on snow.
Well that makes it more interesting! If you didn't have to buy them unless they fit, I'd have a shop mount a set of 275's, then see if they rub. I go to a Tire Rama, and they will mount and test fit to make sure there are no issues. The guy even said we could drive it shortly to test turning and rub.