Clark,
  I have the same Performance ram air valve on my angle valve IO-540 powered ES.  I have my filtered air coming through a K&N filter mounted inside my cowl which just draws the cooling air after it flows down past the cylinders.
  At a Lancair fly-in up in Redmond a couple of years ago I spoke with an owner who bought Carsten Sundin's Legacy from him.  He had the same setup, but pointed out that he was pretty sure that due to the open end of the filtered side of the valve, the low pressure area his filter was mounted in was actually drawing some of the ram air out before it got to the engine. This was my situation as well.  He fabricated another piece to close off the open side of the valve to isolate it when he was operating on ram air and saw some gain in MAP as a result.
  To address this issue on my plane, I ordered parts from Performance to add a second flapper valve in my system which is directly linked to the existing  ram air valve.  When one opens, the other closes and vice versa. Down low I usually see a 1.3" to 1.5" increase in my MAP when I go to ram air, with gradually decreasing gains as I climb higher.  While I didn't quantify it, the MAP rise was slightly less (somewhere on the order of .2-.3") before I added the second valve.
  While I can't explain the small amount of ram rise in the RV setup you referenced, I have to believe that the reason for the negligible difference between isolating the filtered side and having it open is that the filter in that set up was benefitting from high pressure air coming in the intake. Its not likely that much air could go back up that way.  In contrast, my filter is located in a low pressure area, which, I believe, allowed some of the ram air to back flow and escape through the open side of the assembly.
  I'm very pleased with the performance of my ram air setup.  At least from what I've seen, I think the need for isolating the open side of the Performance ram air valve varies depending upon the source of the filtered air.  In any event, the difference is small and the need for the second flapper is really up to the plane's owner.
  Skip Slater
  N540ES