Stefan, how do you find the Arsat? Do you have any sample images?
I had the arsat in my lens-cupboard for a year and never used it (i usualy use the sigma 12-24mm on the 5d for extrem wide angles but want to use my ZD more). only during the last days I begann to test it. my arsat is - even at the corners - sharper than my (old manual) 35mm (which I feel isnīt a good copy). and its angle of view (even defished) much wider than the 5mm would sugest. I start to think that (for architecture/interior) itīs perhaps easier to completely defish the arsat, than to use a non-fisheye-lens with some more complex distortion which might be more difficult do correct.
I will try to find some test-images latter to post. But with most ukrain or russian lenses the variation between copies might be big. for the small price one can afford to buy a few lenses and only keep the best copy.
I think for landscape use the arsat is a very good solution.
another suggestion: for architecture exteriors i often use the mamiya shift with two (upwards shifted) frames which I stitch in PS. you get about the 35mm angle of view with more resolution, more sharpness and no need to correct perspective in PS.
p.s. here are two pics from the arsat -just a quick view into the garden, raw-developer with sharpness on 6. the first is the full frame, the second a 100% crop, still distorted and CA not removed!.