Is the boost return valve closing properly when you put your foot to the floor on the accelerator? Some people have been known to tinker with the linkage meaning that the BRV won't close fully when you floor it.
Also, if you let the car warm up, there is a blue plug to the right hand side of the exhaust manifold (looking at it from the front). When you remove this, do the revs drop or stay the same?
I'm 90% sure your problems caused by lack of boost, though.