I don't know how Novak would fare in another era but he surely is a very adaptable player. It is clear watching his way to the top the he constantly adapted his game to surfaces, to opponents, to equipment, to balls and managed to reach the top. But here are some guesses.
He just isn't a bad clay courter and wouldn't be bad on clay in any era. He slides, he moves fast and he learned tennis on clay.
As for the slowness of the hard courts, I believe that Novak's game might even benefit from speeding them up just a little bit at this particular moment. He would need to improve his serve and net play, which he certainly can, and he moves like a panther anyway. No other apparent weaknesses that would really compromise his performance on any hard court.
Maybe he would never win Wimby in another era, just like Lendl never did. Maybe, or he would still find a way to adapt. Adaptability is his main strength. But again, I say maybe he would never win Wimby, he did get one, who knows if he will ever get another.
Did Nadal really benefit that much from the slow court era, and he must be the most obvious choice when picking a player who would benefit the most from slow courts? He would be a multiple RG winner in any era, and his slam count on other surfaces is still not that impressive for a living tennis legend.
A moot point, all in all.
As for the main topic, Novak might not have surpassed Edberg and/or Becker but it seems he will, by any kind of measures, and it seems it will happen soon