I've just been doing a similar test -- comparing my Sony 500mm reflex with the Tamron 70-300mm on my A77. The reflex came out much sharper, but the Tamron at 300mm was fuzzy enough that I suspect I must have messed up somehow. I'm going to have to do some more careful testing with manual focus on a tripod with good light and clear calm air and not wearing gloves
Years ago I did a quick test of the 500mm reflex against the then new Sony 70-400mm G. They let me take the 70-400mm out of the camera shop and try a monopod shot of a church roof down the street. I tested them with my A77 and the shop's A99. The result was the same with both cameras. The 70-400mm at 400mm was getting a just discernible amount of extra detail if you examined the pixels very carefully.
I've been reading up on the technology of catadioptric camera lenses. The web seems curiously vague about this 500mm Minolta/Sony reflex. Is it a Schmidt-Cassegrain Newtonian with Mangin mirrors?