I am certainly no expert but I think the Bernoulli principle (an expanding stream of air shows a reduction in velocity and an increase in pressure and vice versa) applies if the expansion is generally non-turbulent, which happens if the rate of expansion is around 5 degrees or less.  In my Legacy with a Lycoming engine, air intake on both sides was turbulent.  On the left much more on the right because the baffling angle is much steeper to accommodate an oil cooler (which we installed elsewhere.)  It is fairly clear that no one did any airflow engineering on the intakes.  We got some significant temperature reductions (about 35 degrees CHT) from making some minor alterations to the right side baffling (basically putting in a small ramp that blocked about an inch of the #1 cylinder), installing about a 4" deep cone on the left intake and a deflector to direct left intake air down toward the #2 cylinder.

Once the cross sectional area through which a stream of air passes is increasing at a rate greater than 5 degrees, the flow becomes turbulent and the Bernoulli principle ceases to apply.  Therefore the rapid increase in area entering the plenum chamber should have unpredictable effects on pressure (but good intake geometry can increase pressure before the air gets to the plenum chamber.)

George Braly's observations (as usual based on actual data for which I am most grateful) were very interesting.  Based on them, I think I am going to stop work on the plenum I was building and concentrate on seeing if I can make further improvements to the intake flow.

The turbulence problem also applies to outflow and all the junk hanging from the engine and engine mount make outflow wildly turbulent.  If you can find a way to channel outflow, there should be a noticeable increase in speed and decrease in temps.  I have looked in the engine compartment and concluded that that is easier to say than to do but, it might be a nice project for someone who is looking for a challenge.

The person who really has this stuff down "cold" is Dave Anders who has the world's fastest RV-4.  He used to live in Visalia and now, I think, has moved to somewhere near Redding.  I spent a few hours with him a couple years ago and he could not have been more helpful.