Performance difference between 700x28 vs 700x25 tires? My bike originally came with 700x26c tires so I assume I can easily go to either 25 or 28

Actually the Shimano WH-R540 is 19mm which supposedly doesn't go below 28 but there is 25 on there now anyway and it seems to fit properly.

I assume 28 will be slightly more comfortable, especially at "only" 100psi vs the 125psi on the 25 (though that's max, so it could also be run at 100psi)

Is there any way to calculate the performance difference for the same brand of tires?

ie. Are we talking 0.5 mph loss or 1mph loss?

It looks like 28's also need their own width tube because the 25 tubes are not recommended to stretch that far, so I assume there is slightly extra weight there.
Rolling resistance is complex and not easily calculated. Fortunately, the people at bicyclerollingresistance.com did actual measurements for different width versions of same tire. The short summary is that


Wider tire has lower rolling resistance at same pressure
The maximum and useful pressures are lower on wider tires
When each width is inflated to correct pressure (which is far above 60 psi for 23mm tires), there is very little difference in rolling resistance between different widths.
EDIT: At their test setup, 18 mph and 94 lbs load, the power lost to rolling resistance per tire is 12.9W for 25mm@100psi and 12.7W for 28mm@80psi (pressures picked based on what I use). The total difference, 0.4W for both tires together, makes approximately 0.02Mph difference in speed if everything else (including air resistance, which isn't exactly true) is kept the same.