I think the B-407 was supposed to have a "silent mode" where you could flip a switch and the RRPM would decrease some percent! Donīt know if it is standard or what, probably some 407 driverīs will tell us?
I also heard that the AS332 Super Puma has some feature to change RRPM by switch flick, but that is to avoid some phase interference with LLZ on approaches?
Yes in the 212 and 222 you can decrease the RPM to 97-98% inflight for some performance gain, I have been told the gain is neglible though so I have never done that. I keep the RRPM at 99.5 to 100% more or less.
I believe if I remember right that in the Schweizer 300C there is some mention of 3000 ERPM for extended cruise so thatīs certainly less than 100% RRPM. One could probably figure out the performance gain from the tables, I think there are tables both for 3000 & 3200 ERPM!