To be clear, I'm not an aerodynamicist, nor do I pretend to be one, and I could well be wrong.
My understanding is that if you give the air a chance to expand, it slows down. If it slows down, it using contact with the hot surfaces longer. If it using contact with the hot surfaces longer, it can absorb more heat.
In short, I put my plenum on as close to the cylinders as I could. I experienced high than desirable CHT's. I then raised the plenum and my CHT's dropped.
You're both right, within reason.
Static pressure + dynamic pressure = total pressure. Static and dynamic can be traded back and forth while total remains the same. We typically slow the air, i.e. reduce dynamic pressure, in order to raise static pressure, because we need a pressure difference between the upper and lower cowl volumes. That pressure difference makes air flow through the fins.
There are several ways to slow the air. Offhand, the three most common in our application would be external diffusion, internal diffusion (possibly Bill's internal expansion of upper cowl volume), and impact with a surface.
Note that Bill's plenum volume increase may not have resulted in higher static pressure, yet it may have increased heat transfer. Consider that in a typical GA installation, around half the fin area is not enclosed by baffle wraps. A considerable amount of the total heat transfer is due to random air motion within the upper cowl space, in contact with all the exposed fin and crankcase area, prior to the air actually entering the passages formed by fins and wraps.
Reynolds number enters the picture when we discuss flow
between the fins. A low Reynolds number would indicate laminar flow. The required condition is Reynolds number high enough to guarantee turbulent flow, as turbulent flow considerably increases heat transfer from the hot fins to the air.
Returning to the OP's question, a plenum does not guarantee better cooling. A plenum lid is just a sealing device. The cowl inlets and connections to the enclosed space largely determine resulting pressure, as well as the availability of remaining velocity.
