I totally agree that these engines should not be loaded down with too heavy props. With one exception I've always had my 120-140 engines turn in the 8500 rpm range.
The exception was my 140FZ which I flew for about 500 flights with APC 16-12 turning (I think) about 8000 rpms or slightly less. This didn't give maximum performance but as Dave correctly commented very low noise. This engine, even though it was heavy loaded, ran very consistently.

When I say performance degradation I meant a slight degradation of say 200-300 rpms compared to the engines "best period" peak, not an engine breakdown needing rebuild.

Maybe I am too focused on minor details, I just want my engines to perform at their best ... :-)

I still think that the valve/seat seal is very important to the performance. What is the reason that YS has changed the design of the head, using no brass inserts in the valve seats ?
Is it just a cost issue ?

Oistein, Norway