I think that we have something like that in AH too with different options for different planes, because under some circumstances some planes seem to work better with less or much RPM and less or much manifold.
Is it my impression only?
quite right. eg. the spits' merlins give much better cruise endurance at reasonable speeds if you keep full throttle and wind the rpms down until you reach the desired boost (just like the real thing.) this does not work well in a jug (just like the real thing). I very much doubt many people fly by-the-book this way, but its impressive that its modelled so you can
I'd suggest picking up a copy of the pilot's notes for your favorite aircraft and trying the by-the-book settings for economical flying, my guess is they will generally work.
if you mean settings while in combat, always use max rpm. this gives you max thrust at full throttle, and max drag to decelerate the aircraft when off the throttle