After switching to 30% fuel this weekend I think my quest for ever-larger engines is over! 15x4W @ 10500, put a 16x4w on which brought it down to 9300 rpm.

DM, I don't think the engine has vibration issues, but if you idle it way down to 1300 rpm on a solid mount things might break. That's when mine broke, playing with the idle capabilities. It has regular hinges in the rudder now, working fine.