As a Minolta fanboy, I've used and can report on the following:
Kiron 28 mm f/2.0 MC
A little soft wide open with low contrast due to longitudinal CAs ("haze"). Sharpness and contrast pick up at f/4, but never reach the performance of the Minolta MD 28 mm. Very strong lateral CAs (red/cyan borders on contrasty edges).
Minolta MD 28 mm f/2.8 (MD-III)
Surprisingly sharp and contrasty already wide open without the typical longitudinal CAs common to vintage lenses. Extremely sharp from f/5.6 on with great contrast. Some lateral CAs.
Minolta MC W.Rokkor-SI 28 mm f/2.5 (MC-II)
Didn't test this one in detail, because the slight radioactivity pushed me to sell it again. There's thorium oxide in one of the lens elements, which is not dangerous if you don't sit on the lens all day. But the idea didn't feel comfortable, so I sold it.
However, this lens has excellent optics. From what I remember, contrast and sharpness wide open were practically on par with the MD 28 mm f/2.8 (!). Can't say anything about CAs, though. Stopped down, this lens is exceptionally good. The glass had a visible yellow color cast. I'll probably get one again for a short test