George,
I have not made such measurements.  The upcoming Lancair fly-in would
present a great opportunity for checking several aircraft types.  I
wouldn't expect to see any real difference though.  The plenum top is
trapping heat just like a regular cowling would.  It just sits a little
lower.  The standard cowl would allow better conduction through the skin
to the outside, but the rather thick composite skin is a good insulator.
 Based on touch I would guess the surface temperature of the original
cowl to be in the 140-160 degF range.  I would expect to see most of the
heat escape through natural convection (in truly still air) through the
inlets.  The wires would have been exposed to much the same environment
in either set-up.   I'll bring my thermocouple reader to the fly-in and
look for volunteers.  This is a measurement that is easy to make.
BTW, I measured magneto temperatures in the 360 several years ago.
After flying around for an hour with several take-offs and landings, I
could not get the temperature up to the normal operating range quoted by
Unison.  The highest temperature occurred after shutdown and the
subsequent heat soaking.  I didn't stick around waiting for the peak as
I was only interested in operational temperatures at the time.   I have
the long engine mount.
I will report back with results.
regards,
Chris  


Christopher Zavatson
Mobility Technologies
United Defense
(408)289-4329