Is it fair to characterize the post-war Simson, J.P Sauer and Merkel as good, better, best, at least along similar models, with the fit, finish and detailing creating the difference?
I don't think it would be fair. If you look at the print catalogs put out by the DDR for Suhl-made shotguns, say from the 80s, they will have the same gun with the Merkel name on the one page of the catalog, and the Simson name on the facing page. So, like I said, "Same gun, same design, same factory, same workers, different name". I think any variation in quality is the result more of the day of the week than the name on the barrels. Remember, these were all made by Germans and they didn't stop being Germans merely because of some change in government.
As to Sauers, remember there were East German Sauers and West German Sauers, the former being the "original" factory/company in Suhl taken over by the East German government and consolidated into the other companies in Suhl. The West German Sauer was formed by the Sauer family/owners when they jumped the fence to the West and restarted the business from scratch. I think it's fair to say the East German Sauers are the equal of the other East German guns and they and the West German Sauers are at least as good.
In the end analysis, though, this kind of hair-splitting is like arguing which shot size is best for pheasant.
