Bro, are you going to deny they benefited from a slower high bouncing surface at Wimbledon??
Realistically I think Roger would have won 9 or 10. More than that would have been ridiculous. He would've been slightly more vulnerable to big servers/hitters in his prime on fast grass but point blank Djokovic and Nadal would've been next to no threat on the prior grass. I don't even think they'd have made more than 1 or 2 finals
Understood that the change in surface benefitted players like Nadal and Djokovic and Murray. However, you can't just translate that to more wins for Fed. I made the point above that you don't know how these top players would have adapted, if that was the hand they were dealt, given how much they adapted to the challenge of Roger, and then Nadal. But rather than say randomly that Roger would have won more at Wimbledon, let's pick that apart:
2008: I know you think that Nadal should never ever have beaten Roger on grass, anyway, but you also claim he was still suffering the effects of his mono, and Rafa was well in his head, so that might not have changed, anyway.
2010: Roger lost QFs in RG and QFs at Wimbledon. Would he have done better v. Berdych with a different surface, and still made it 2 more rounds? That's surely debatable.
2011: You still have to talk him past Tsonga and Djokovic, then Nadal.
2013: He had a crap year.
2014: He played a good final but Djokovic was very much trending, so that still a 50/50, even on different grass. (And he was 32.)
2015: He played a stellar SF v. Murray, then a limp final v. Djokovic. No different grass was winning him that match, and given Djokovic that year.
2016: (Didn't play.)
So I don't see how you really get him more Wimbledon's than he won anyway. I know you'd like to claw back 2008, but he had mono. The only other reasonably argued one is 2014. The rest are not wholly down to surface changes.