First of all, it's a huge chimney of a lens!
On a 6x7 it's almost impossible to use without a good tripod.
The level of performance on film, especially a 6x7 frame, is quite high, compared with affordable alternatives in the same focal range.
On a high density sensor it shows its limits. Fringing (not extreme but very visible) and lack of sharpness if you don't stop down to f/11.
In this kind of lenses the damage inflicted by diffraction is way lower than the effect of undercorrected aberrations.
Which means that f/11 and f/16 are working apertures.
All in all, I would not be surprised if it almost matches the performance of the Pentax (6x7 version).
Mirror lenses usually show no fringing at all, but most of them lack sharpness. The sharp ones are very, very expensive.
I have seen soviet mirror lenses adapted for the Pentacon Six. I guess they should cover 6x7.
I don't know if it's worth the hassle, though.
One possible alternative are the long focus achromats, like the Leitz and Novoflex.
The design allows for a huge circle of coverage, but the performance decreases a lot towards the borders.
Maybe they would be usable for birding, with the main subject positioned close to the center of the frame.