Biggest problem with Jenson is the limited amount he can do with a car that is not to his liking.
Alonso, Vettel, Hamilton etc don't suffer to the same extent he does as they can drive around problems better, whereas if Button's car is out of balance, tyres can't warm up etc, he is nowhere. Because of this he then can't change the setup to find a compromise as that doesn't work for him either.
A midfield team doesn't need a setup perfectionist, they need someone who can drive the car no matter what it is doing. Jenson is not that person. He also seems resigned to where he is at the moment and lacks motivation, that can't be good for the team either. I'd be surprsied if he is still at McLaren next season. As said above, with Boullier's connections to Lotus, Grosjean to McLaren may be a good bit of business.