Hi Kjell Olav.
I had a hacker C50 (NOT the Competition version) hard mounted in a F3A model. It worked OK.
When I later moved the motor to a new model, where I used rubber mounts (4 pcs, 10mm dia. x 10mm long), it was noticeably quieter.
I have no measurements, but it was easy to hear the difference.
I will soft mount the Hackers from now, (even the competition, if I ever get one) while I am happy to hard mount any outrunner.

Regards,
Magne