The pressure ratios for the turbo charged car are diffrent for the 2 diffrent elevations. So you could be loosing power just because you are going into a diffrent area of your flow map.

Pressure Ratio (ATM+Boost)/ATM

(14.7+10)/14.7 = 1.680
(13.25+10)/13.25 = 1.755

Higher pressure ratios will move your point higher up on the compresser map. Most of us will suffer from lower efficency, unless we are running very large turbos. We will probably even hit the surge line upto a higher RPM then we would at lower elevation.

Not only that, but your VE (volumetric efficency) is also lowered at the higher altitude, even on a turbo car. You dont have 100% efficency in getting all of the compressed air into the cylinders. The cylinders are starting out at a lower pressure then they would at lower altitude.

So I think what Elijah said, although somewhat true, does not take into consideration all of the factors that go into determining power loss. Also the correction during the time that you are off boost or when boost is coming up or falling.
The system is so dynamic, that you cant just use one boost number to find the correct answer.

Eric