I've always felt one of the best ways to figure out how much a player has declined is to imagine how he'd go against his peak self.
2011 Novak(peak) would beat 2015 Novak in 4 sets everywhere I think. Something like 6-3 3-6 7-6 6-2
2008 Nadal vs current Nadal would be hilarious. Straight set beatdown : 6-1 6-2 6-2
2006 Federer vs 2015 Federer - I expect the current version would sneak a set on the serve alone. The rest of his game is no match, however. Something like 6-1 6-7 6-3 6-3
2011 Novak(peak) would beat 2015 Novak in 4 sets everywhere I think. Something like 6-3 3-6 7-6 6-2
2008 Nadal vs current Nadal would be hilarious. Straight set beatdown : 6-1 6-2 6-2
2006 Federer vs 2015 Federer - I expect the current version would sneak a set on the serve alone. The rest of his game is no match, however. Something like 6-1 6-7 6-3 6-3