It is puzzling and since you clearly know what you’re taking about I was interested to hear your thoughts.
Both have CS props.
I withheld some info in that my friend has over many years increased his Monsun speed through little detail fairing changes, I’ve flown his plane and it is substantially faster than book speeds. 
However, the abrupt windscreen angle can’t reasonably be changed.
Tail volume (RVs have no more than they need) and cowling design come to mind as well. The kind of things American Aviation changed when they created the Tiger via aerodynamic cleanups.
Regardless I think Richard Van Grunsven and the designers at Vans are very good at what they do.
Very interesting, I will study this! Sounds like high altitude and thereby low IAS is the place for the Monsun.
I believe the Comanche uses about the same airfoil as the Monsun, but I’m not smart enough in the area to say much more. The Monsun wing visually looks thin relative to the RVs visually fatter section. This puzzles me too. It would be great to learn more as the relative efficiency of the RVs confounds me.
Antonio wrote:
the low-lift (high-IAS) drag of the latter is twice that of the former…
I checked some web sources and it seems more like 25% delta, but nonetheless significant. For reference, such a speed delta (160 vs 135) , all else being equal, needs about a 35% drag reduction
Yes, even if that were only half the drag of the plane it’s still 12% more total drag. Then add in more parasitic drag and the story is clearer.
Peter wrote:
An example I have often posted is that the TB20, the Cessna 400 (a.k.a. TTX), the SR22, and even the DA42 all do ~140kt IAS at 5000ft at 11.5 USG/hr (I tested all these myself)
That is an amazing apparent coincidence…you can’t beat physics
Peter wrote:
all do ~140kt IAS at 5000ft at 11.5 USG/hr
That is indeed a very wise use of the sign ~ by @Peter I bet none of the aircraft was flying at the same speed, nor the same altitude, and even less with the same FF. I used to do some formation flying, and numbers comparison on very similar ships pointed to quite some cumulative disparity, despite flying in the same atmosphere at the same whatever speeds…
Antonio wrote:
nobody seems very interested in Carson speeds
To the dismay of some of my flying companions, I’m usually flying at very slightly more than Carson 
I was in all personally.
Peter wrote:
I was in all personally.
no doubt Peter… but probably not at the same time, nor space, and not reading off the very same instruments for all of them
