Screenwriter Tom Mankiewicz (co-writer for I, uncredited like Donner on II) recalled a story that when II came out, some particular reviewer praised the movie and Richard Lester for extracting much more impressive performances from the actors. In specific, Gene Hackman's acting was praised by this particular reviewer as a marked improvement, thanks to better writing and better directing. Mankiewicz wrote in a letter to the editor, which was never published or even acknowledged, pointing out that every last bit of Gene Hackman's performance was directed by Donner, from his script. Hackman apparently refused to work on Superman II at all after the Donner shakeup, and they had to edit around him, occasionally using wide shots and a (very impressive) voice double.
Incidentally, Brando legally sued the Salkinds to have his footage removed entirely on the grounds that they'd only contracted and paid him for one film. (The Salkinds were notorious for this tactic already, having done it with The Three Musketeers / The Four Musketeers). John Williams dropped out soon afterward but I don't recall if he specifically cited these reasons. Hackman, Brando, Williams ... these guys showed the Salkinds what true power in Hollywood was about. And history has been quite kind to them as a result.
Donner has publicly stated numerous times that he wants to do his own edit of Superman II. However, given that the unused Brando footage is now going to be used in Superman Returns, I don't know if it would ever be included. I do think that II sorely needs Jor-El's ultimate sacrifice to explain how Superman got his powers back. Given that Warner absolutely LOVES Donner (he's made them billions from many of the films mentioned earlier), I'm surprised this project didn't start the day after they bought the rights back from the Salkinds.