I'm in the same boat as JC--never heard of an "official" rough cut. But, yes, many hours of additional scenes were filmed and discarded--including, for example, Paulie and two heavies beating up the scum who ruined Bonasera's daughter; and the scene JC referred to. I strongly urge you to get Harlan Lebo's "The Godfather Legacy," which many of us here regard as the GF Trilogy Bible. The book has photos of that scene where Michael kills Fab. Also has a reproduction of the FBI family organization chart seen in the Senate hearing.

BTW: Have you ever seen the "Saga" version and/or the "extras" disc in the DVD boxed set? Both contain the deleted scenes from GF and II.