Artisan,

Unloading and lean runs are NOT an R/C myth.

When the load is reduced on the engine; whether from unloading in flight, or from using a smaller prop, RPM increases.
Unless your engine is already propped at peak horsepower (like a muffler equipped, sport .60 on an 11x7 prop...), every rise in RPM is also a rise in horsepower, even if torque is in a decline.

To produce more horsepower the engine needs more fuel.
Even though faster airflow through the carburettor increases suction, the main needle needs to be opened further to maintain the correct air/fuel ratio, as RPM increases and horsepower is too.

This is why it is imperative to enrich the main needle, with the nose up, by 100-200 RPM (more with the plane horizontal).

Extraslim,

Make sure you have the shortest possible fuel line from the tank to the carburettor (or to the needle). You didn't bother to tell us what make of engine you are using, but a 2 blade 10x4 is too small for your engine. It may not over-rev, but your plane will sound like you are driving in first gear all the time.

Long fuel lines do cause transition to be hesitated, as the plane accelerates. From the descriptions you wrote, it hesitates on pickup, not sputters. Fuel going to the engine must also overcome acceleration forces. Maybe the immediate initial acceleration from your low pitched prop is a part of the reason for the hesitation.

I suggest to enrich the idle mixture 1/8 of a turn AND to change to a 10x5, 11x5, or a 10x6 prop.

Tell us about your results.

Sincerely,