You could plot the Merlin III boost ratings on a chart and they would pretty much follow a straight line. let's remember that 16lb of boost is pretty close to 62in of MAP.
The Merlin III might have been able to generate over 70in at sea level

Question is wither the engine would run for more than a minute or two.
The higher you go the less absolute pressure the supercharger can make.
Some of the Melrin XX engines were rated at 14lbs in low gear and 16lbs in high gear but I can't recall the dates at the moment. At some point they may have been allowed to use 16lbs in low gear?
However I don't think that the 16lb rating for any Merlin was used in 1940. The CAM ships didn't go into action until the summer (or very late spring) of 1941.