Yes of course I factored this in. Edberg vs Muster and Fed vs Nadal comparison is difficult because of the poly strings that allows the spin that Nadal is able to create. Fair enough. But lets talk about Fed's slice vs Edberg's slice. Edberg's was much heavier and penetrating. Guys who played against him with big forehands (Lendl, Becker, Agassi, Courier, and Sampras) rarely were able to attack it compared to guys today (Nadal, Soderling, Joker, and even Murray) who are able to attack Fed's backhand slice. Nobody except Rosewall (I never seen it but I'll believe those who has) had a better slice backhand then Edberg.
First, how much more spin do you think Ralph generates with his string than he could have with the string available in the 90's? 5%? 10%?
Second, I agree with your assessment of Edberg's vs. Federer's bh. And, I think that this difference is a fairly universal distinction between most backhand slices today vs. pre-2000. It seems that, today, players are hitting more of what Tilden would distinguish as a "chop" rather than a slice. In the past, most player's hit mostly slice backhands. But, they were drive-slices with a much flatter swing path, a rolling backspin that resulted in a more powerful, penetrating shot. Today, it seems all backhand slices are executed with a much more drastic downward swing path, imparting a much more severe backspin than a pure slice would generate. I'm not sure why this change occurred, but, it's quite obvious to me.
In any event, IMHO, Edberg's backhand was better than Federer's to the extent that it was much more reliable. Edberg's backhand was money in the bank. Federer's backhand can be spectacular, but also, can be erratic it terms of clean ball striking and consistent depth and accuracy, and has successfully been expoited. I don't ever recall Edberg's backhand letting him down the way Federer's has.
As for great slice backhands that were as good as Edberg's, in addition to Rosewall I would include Laver, Roche and Emerson. 5 of the best 1hb's of all time. Among these, only Rosewall didn't have an equally great topspin backhand. But, Rosewall proved conclusively that he didn't need one.