I would have thought that if you had carbon buildup the compression would be lower due to sticky valves not closes as they should.
I have a 90 hp Mercury and my compression is supposed to be at 220 psi which I think is super high but that is how they designed the motor.
no "valves" on an outboard motor.
my 300 drag with o-ringed 17CC heads made 260 lbs compression.. had to run 112 octane fuel or higher.
that motor hit hard at 11k rpms.
a 90hp engine runs about same compression as the bigger ones. around 110- 120 is it.
imho. compression over 140-150 needs race gas.