I know that if you export 100% quality, is RGB. I know that if you export 75% is YUV 422.
From Compressor I have arou 95-98% quality in Photo-JPEG.
BTW where are you seeing the Gamma Shift? IN FC, QT Player?
The shift is visible only in FCP and when rendered out of it. Other than that Photo-JPEG footage from Compressor plays with same as native gamma in QT.
But when I export HDV from Compressor with ProRes in both QT and FCP I get shift to the darker side.