I read a little about the Russian ‘Bear’ aircraft recently. IIRC the comment was that although the contra rotating props turned at a maximum rpm of only 750, each prop was 16ft in dia and the prop speed exceeded mach 1 by about .08.
Still it was amazingly fast with a max speed of 585 mph. No idea about efficient it was, or how much noise those prop tips made!
Regards,
White Bear.