" Yes, and unfortunately very addicting. When I had my 2003 SVT Cobra Mustang, MPG varied wildly depending on how much I was pressing down on the go pedal.
Avg: 16 MPG around town.
Best: 26 MPG from NY -> MA @ ~75 - 80 MPH
Worst: 7.5 MPG around town. :-D
As others have pointed out, as you have a fixed unit in which you are compressing, you are generating a bit of heat. The way to make more boost is to spin it faster, which reduces efficiency, heating things more. A air-to-water intercooler is ideal in these setups, but require custom intakes, which if I read it right, has been done here. It should make the car even more of a beast.