Yes the GM X66p is desined to decarbon modern engines with multiple exhaust O2 sensors.

I wouldn't worry about your compression if they are all the same. There is so much difference from gauge to gauge and technique to technique that numbers aren't really the issue. BTW I had an instructor tell me once that the relative pressure should be the compression ratio times the atmosheric pressure in psi.

Besides if you have a ring problem causing your consumption problem it won't show on a compression test as that checks the compression rings not the oil ring.

If you have equal affects on each cylinder I would condemn the valve guide oil seals even if new. It is the weakness of these motors. I have also seen techs place the seals on the wrong valve thinking they are the same. I also only would use factory valve guide oil seals.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician