Use the tire manufacturer specs for dual spacing. That cannot be calculated with an generic tire compare tool - it's an engineering analysis. In all probability, though, those two sizes will be dual-spacing compatible if they are the same manufacturer.
The extra height of the 80's will increase the diameter slightly, which in turn reduces the rev/mile a tiny bit, but probably not
enough to be concerned. Again, the revs/mile comparison will tell you the exact impact of the speedometer/odometer. The small amount of extra height should not be an issue in the wheel well either, unless it is already very tight there.
No problem changing fronts and rears at different times, as long as all tires on the same axle are the same.
Would anyone make the switch for ride comfort alone if the existing tires are in good shape?
Absolutely not. I sincerely doubt you will be able to tell the difference. A couple of psi more or less in the tires will have far more effect than a 0.10 difference in section height ratio. For a 275 width, we are talking only 1/10 of an inch!