Yes, his EFFICIENCY and EFFECTIVENESS got worse, with only a few exceptions. For the sake of fairness, I'll include only prime years of 2005-2012
-2005: season PER 22.3, WS/48 0.240; playoffs PER 24.8, WS/48 0.260 (this was GOAT playoff Manu)
-2006: season PER 22.4, WS/48 0.234; playoffs PER 21.5, WS/48 0.154 (dribbled the ball off his foot to end game 3 against Dallas, fouled Dirk then missed the game-winning layup in game 7)
-2007: season PER 24.1, WS/48 0.246; playoffs PER 21.9, WS/48 0.204 (absolute beast in the RS)
-2008: season PER 24.3, WS/48 0.232; playoffs PER 17.1, WS/48 0.112 (maybe his best RS ever, but playoffs were marred by inconsistency after an ankle injury in round 1)
-2009: injured in playoffs
-2010: season PER 22.5, WS/48 0.216; playoffs PER 21.4, WS/48 0.149
-2011: season PER 21.7, WS/48 0.195; playoffs PER 22.3, WS/48 0.155 (by far our best player in an embarrassing playoff sweep, despite playing with a broken arm)
-2012: season PER 24.1, WS/48 0.257; playoffs PER 17.0, WS/48 0.120 (again, inconsistent play with flashes of brilliance; brilliant in game 2 against the Thunder, but completely disappeared in the deciding game 6)