Power issues
 

I am driving a 2009 XKR and am about to have the supercharger uprated. Just before taking it to the garage however I noticed an issue. From 4000rpm upwards the engine is really struggling to get any pick up, and seems to be sputtering. Also if you dump the gears and put your foot down there is a nocking noise almost like a belt or something has come loose. Pre 4000 rpm though everything seems ok. When I took it to the garage they found one of the air meters had broken and had fixed it but the problem remains. Now they are saying that they have to strip the supercharger and that it will be expensive etc etc....I'm sure you know the deal. Has anyone had a similar issue before I get landed with a heft bill.