Performance difference between 700x28 vs 700x25 tires?
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.