What would MP be with nothing at all connected to the throttle body? The
IO-550N engine air intake is located in the high-pressure side of the intake
plenum. Common wisdom says that a ram air duct facing into the slipstream
boosts MP, but we already have two big scoops, namely the cooling inlets!
Pressure rise from a ram air duct should be nearly identical to the air
pressure inside the cowl. Anyone have any thoughts?

Adam,

 

Of course.  In my wee Lancair at about 183 KIAS (typical cruise) I see about 12" H2O above ambient static pressure in the upper cowl.  That is approximately  .85" Hg (MAP).  However, that does not take into account any more than engine cooling air being bled off for the induction system or oil cooler since they have separate intakes.  You are talking about the superior intake setup for Continental engines - cold air in to cold runners - rather than those of us that suffer with induction air being run thru the oil sump.

 

At 183 KIAS I should be seeing about +1.5" Hg added to the MAP because of a dedicated ram induction port.