Hello; I figure that in every other installation of an internal combustion in the world, the engine is anti-vibration mounted. They must know something about it. Only the occasional motorcycle engine is rigidly mounted.

If the engine is turning 10.000 rpms, whether it is soft mounted or not won't prevent the air passing over the prop, or decrease the amount of power produced.

What anti-vibe mounts will do is minimize vibration damage to the airframe and radio reciever.