Colyn,
<<    ..any idea what the performance improvement is for adding that?>>

I don't know if there are any Continental driven Lancairs with before and after numbers.  I have one case study with a Lycoming O360.  

I made the switch to a plenumed system on my L360 about 16 months ago.  Having flown with the original baffle system for 6 year, I had lots of data for comparison.  In general, the hottest cylinders dropped 30+ degrees.  I also gained 5 kts in airspeed.  Obviously a system could be optimized for speed or temperatures depending on one's goals.


Related data can be found in NASA report 3405:
"An Experimental Investigation of the Aerodynamics and Cooling of a Horizontally-Opposed Air-Cooled Aircraft Engine Installation"
http://www.n91cz.com/cooling/Copy_of_NASA_Cooling_Report.pdf

Chris Zavatson (firm believer in plenums)
N91CZ
360 std
www.N91CZ.com