I think for my setup, there's room for improvement. I find my stars to be a little bloat and that could be a combination of the environment I take my photos (I'm in Tokyo so I go to Chiba which is about 1-1.5 hrs but still distant factories with hot/cold air rising/dropping so a little turbulent), and the A7S as good as it is for low light has its own issues.
i.e. I don't think I'm getting the best out of my 250CRS.
However I am shooting full 35mm frame so edge to edge it's stars are pretty sharp. Mind you the 250CRS has the corrector built in already. I don't know whether your proposed 250 has a corrector or not.
Light falloff-wise, it does vignette somewhat so flats required.
NB: Using the 250CRS for visual though, really nice
.