I just had a look at dyno data, 31" / 2400 and 16.5GPH was about 220-224 BHP.
I think you did another POH MYTH BUSTERS program
Seems to me at least the POH is predicated on the wrong compression ratio….or as we have just decided to call it the expansion ration more correctly
They have used a calculation based on the typical 8.5-8.7:1 engines. Oops!
Will have a chat with the guys after breakfast.