Maybe the easiest thing to check for is a leak in the plumbing. Even without black smoke evident it is probably still a possibility. The quickest and easist way I have found to look for leaks is to pressurize the system to about 15 lbs. using my air compressor. I use a master cylinder cap with a Schrader valve to bleed my brakes (using a bicycle pump). It happens that the cap is the correct size to insert in the hose coming off of the turbo. By clamping the cap in place and adding 15 lbs of pressure (I turn the regulator on the compressor down) I can very quickly find any leaks by the tell tale hissing of the air escaping. At least you could eliminate that possible performance robbing problem.

Randy