i know of
a few who have tried to use the IVO in flight adjustable prop for
high speed flight, and found it less than satisfactory. Since the
inboard portion doesnt rotate as much, it must produce some "flat
plate" drag.. anecdotal reports of a 10 mph penalty on a 200 mph
aircraft are what I recall.

Thats at least partially true of all inflight adjustable props. For a
particular speed and power there is an ideal shape that can be achieved
by fixed pitch props. The adjustable (CS) props design their shape for
an average effect over the speed range. But the adjustable prop still
gives a decent high speed efficiency while giving that extra safety
factor at takeoff and landing - takeoff like a slingshot and you can
really slow down the plane on landing with low pitch setting. So you
have to take your choices - can't have everything.
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SQ2000 canard http://www.abri.com/sq2000