I think we have seen the best but I certainly don't think he will decline as a player.
I think there are a few more attributes people will start to notice and question more because he isn't in the salary cap rorting Storm side or the most dominant origin side in history but that doesnt necessarily mean he will be worse.
Moving from Melbourne (Slater, Finch, Cronk & Smith) and QLD (Slater, Lockyer, Thurston & Smith) to Souths (Wesser, Sutton, Sandow & Luke) is going to make any player look less than what they have looked in the past so in that sense maybe we have seen the best unless he can find a way to a) get more involved or b) find a new position (most suitably fullback imo).