Larry,

I think one of the original factory 320's had such a door (maybe not, it was
a long time ago).  I cannot remember seeing another one.   I also don't
remember any performance numbers relative to the door.    

I was at a 320 (360?) 1st flight not too long ago, the builder had created a
fairing on the right side of the fuselage, the non exhaust pipe side, to
neatly meet the cowling.  Don Goetz had him remove it after the first hi temp
flight.  Subsequent flights showed a dramatic drop in temperatures.  So?  So,
it is not a good idea to reduce the size of the cowling exit air opening
since it is small to begin with - that is if you have the foward mounted
throttle body and, thus, a smaller bottom on the bottom cowl.  

I doubt that the opening is a vertical gusher with the same consequences as
the hi speed exhaust pipe air.  There are ways to shape the inside (and
outside) of all the exit air openings to make them more efficient.  Lancair
found that flairing the opening created a lo pressure area rather than have
the air tumble at the trailing edge.  That allowed more air to exit.  I think
later cowlings just had larger openings - the so called "Dolly Parton"
cowling popular on the IV and Legacy.

Scott