Rob,
I owned the 2,8/25 and used it quite for a while before I sold it afther I got myself a 2,8/21. The 2,8/25 is one of the oldest lens designs in the Contax-lineup and goes back to around 1970. It was developed for the Contarex at a time when 25 mm focal length was looked upon as really super-wide and Zeiss did quite a good job on it. Distortion and sharpness were so much better than average that even now, 30 years later, the lens can compete with nearly any other product available. Only if you compare it to some newer Zeiss lenses it might show the age of its design.
But I think that in order to reach its optical limits you really would have to shoot under optimum conditions, on a tripod and with Velvia slide-film (or use BW film and do your own enlargements). Otherwise the film would be the limit, not the lens - at least as far as sharpness and resolution is concerned.
Color rendition I have always found to be flawless, slightly warm as with all CZ lenses that I have owned.
Distortion of the 2,8/25 is within the limits of what you would have to expect from a lens of that focal length. Remember, there is no wide angle lens in retrofocus construction, which is needed for SLRs, without distortion, and the wider it gets, the more distortion becomes a problem. If you mainly want to shoot architecture, I would however recommend the 2,8/28 which has almost the same field of view and much less distortion. For landscape, the 25's distortion should not be much of a problem.
FWIW
Marc