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hi Martin
hmm... I'm thinking your expectations were a little high :-).
Camera lenses are not like telescopes with fixed apertures. An f2.8 imaging scope is expected to produce sharp stars to the corners of its imaging field because f2.8 is all it can be. Some scopes can even deliver on that promise.
Camera lenses with variable aperture offer a wide open aperture that is a bit of a compromise.. if you're shooting portraits or whatever, you'll often accept a bit of softness in the corners (maybe hardly noticeable on regular images) in exchange for the awesome amount of light you get at f1.2 or whatever it is.
With my 50mm f1.4 lens, I typically shoot widefield images at f3.2. That's about where it becomes tack sharp on my 40D. Using a mono sensor is even more demanding. There are not many optical systems that can deliver tack sharp stars from a wide field of view at anything much faster than that.
If your 1.2 lens can't do any better than f3.2 (noting extra demand of mono sensor) then you could sell that and save yourself money with the 1.4 lens. For most peope I think that would be better value.
I had the f1.8 lens previously and had serious image tilt with it. Whether it came like that or was the result of a knock I'll never know. The tolerance on tilt at f1.2 is stupendousy small.. I don't how anything in the camera can be manufactured to that kind of tolerance, let alone all the pieces working together.
Can't tell a lot from your sample image, but the star bloat/halos look like what I expect wide open. Show me f3 and I reckon we'll be talking. You'll still get deep exposures real quick!
Phil
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