IIRC, for a constant MAP, the air going into the cylinders will for typical compression ratios be at a higher temperature due to a higher degree of compression, even though the ambient temperature is lower. I will have to redo the calculations to be certain. This would explain the fuel flow as the the air mass will be lower, i e a lower fuel flow for the same mixture.
The higher degree of compression will likely (as in 'I don't know but my educated guess is...') mean a higher amount of HP eaten by the compressor even in a supercharged engine. For a turbo, this would definitely be the case as the RPM would be higher, resulting in a higher backpressure against the turbine.
Then, as most people point out, the big gain is in not having to push the exhaust out against sea level ambient pressure.
Why some people talk about flying above the throttle full open altitude is beyond me, as the MAP will then drop off...
Cheers,
Fred