maybe this will help a little.
I live in SF bay area and brought my boat to lake Tahoe (6,000 ft) last summer. I have a carburated 7.4 mercruiser on my Baja 232.
I called many people in Tahoe to ask what to do when I go to high altitude. The consensus was to not mess around with the carb, but to just prop down a size or two.
So I normally have a 23P prop, and i went and bought a used 19P prop for the week.
The bottom line was the prop was too low pitch, and I should have stayed with the 23p and made NO adjustments of any kind.
The boat ran the same up there as it does at home, and I would suggest that he try it first and see if his doesn't run the same also.
Everyone said not to mess with the carb unless thats where the boat will stay permanently.