I been into fast boats, motorcycles, and cars for years. Hi compression engines do require high octanes to prevent engine damage from the fuel air mixture exploding on its own rather than the spark from the plugs. Almost as if it acts as a diesel engine. You normally hear the pinging from the engine and if left unchecked it will destroy the engine. Most of our marine engines are lower compression and have limited spark advance and do not need higher octane fuels.
Octane prevents pinging but does not really make more power on its on.
That is why the HP output in the test is very similar and the variations could just be due to changes during the test conditions.
So save your money and run the octane fuel recommend by the engine manufacturer
I will add the quality of the fuel could also make a big difference, does it use MTBE, ethanol, or other additives. In the north we have winter vs summer fuels that are easier to vaporize in cold weather.
I run 91 octane marine fuel for the fact that being non ethanol it is much better on the rubber and plastic parts in the boats fuel system.