It looks like the forward speed (thrust speed) with forward tilted nacelles is simply too slow. The wing loading is very high for a model this size. 3kg vs a 1m Stub wing is Gee Bee territory. Thus you will need Gee Bee speeds. Which it didn't even nearly achieve in the vid. Higher thrust speeds would likely require more pitch angle in the blades (instead of simply more rpm) if you don't want to mess up efficiency in cruise flight. Challenge might be to keep enough static thrust and efficiency for hovering. But it is a really huge achievement to make this thing work as an RC model.