Bill,
<<Am I correct and would installing a separate NACA duct to supply air to the cabin heat valve result in lower CHTs?>>

Why not perform a quick test to check the impact.  Go on a reference flight and record; ALT, MP, OAT, RPM, EGTs, CHTs, and the price of avgas.  Now block off the cabin heat inlet.  You'll also want to disconnect the tubing at the heat muff so that you don't get a bunch of stagnant hot air in there.  On the test flight reestablish the same flight conditions as closely as possible and take you readings.

Chris Zavatson
N91CZ
360 std.
www.N91CZ.com