Paul raises excellent questions.
"I'm still a little confused as to why the larger reservoir is needed unless it is simply to assist slowing the flow and providing more even coverage of the cylinders."
That is precisely why you want as large volume over the engine as possible. Goal: to reduce free stream velocity much and efficiently as possible so that static pressure is maximized and momentum of flowing air is minimized on top of the engine. This creates the best, most even distribution of air pressure and flow over the cylinders and heads. It is hard to do well in the limited space available with front entry flow.
And it is usually also helpful to lower velocity through the inlet rings, like 50% or less of the free stream velocity. Otherwise you have to slow high inlet velocity inside the cowl if it comes in fast through the inlet rings.
The slow down in air speed can be outside (a somewhat blunt front end with larger inlets) or inside ("pointy" front end with little inlets). Little inlets need to be followed by a diffuser to efficiently slow the air flow, hard to do in a short space. That is why it is usually beneficial to have larger inlet area and careful design to prevent external air flow separation from air "spilling" out around the inlets. Inlet lip radius and cowl contour around and behind the inlets become important.
"A plenum is a smaller reservoir and I'm guessing the lower [leakage] losses of a plenum are dramatic compared to [leakage] losses in a normal cowl?"
Yes. A plenum should have as large a volume over the engine as possible within the confines of the cowl for reasons noted above. Too small and the velocity inside is too high and you get friction losses. Bigger volume - better. But ... Why use plenums? Because conventional baffling installations leak like hell. It all depends on attention to detail and the ability to seal a vibrating engine against a stationary cowl.
Here is the basis for concern: In the 80's Miley et al did NASA-sponsored work on cooling. Their test plane was a Piper Turbo Aztec with turbo 0-540 engines. They made lots of measurements in flight and on the ground. They went so far as to connect the engine in the airplane and inside the cowl to a dyno (no prop) with a long shaft and hooked up a big blower to pump air into the cowl via tight fitting ducts sealed into the cooling inlets. This permitted them to make a careful measurement of the cooling air flow actually entering the cowl inlets.
Findings: if the engine received 100% air flow over heads, cylinders, and oil coolers as required to keep temperatures under control, the ACTUAL air flow into the inlets was 150%. In short, 1/3 of the total air flow entering the cowl inlets leaked AROUND the engine and ended up in the lower cowl having done - nothing - except create additional drag from momentum loss associated with the leaking air.
So the idea of plenums as verified in that report (8 megabyte file, long complicated technical report) is to minimize leakage.
Factory baffle installations of that period, and in many cases still persisting today, are simply awful. Leaks everywhere. The Cirrus is best of class for current factory engine installations.
A good plenum with a good fit between plenum and inlets to accommodate engine movement yields better use of cooling air and lower leakage and thus lower cooling drag. It also produces greater pressure drop across the engine itself and thus better cooling. Why? Leakage around the engine helps to pressurize the lower cowl since more air (engine cooling air plus leakage air) has to escape out the lower cowl exhaust ports back to the atmosphere. More pressure below engine for same pressure above the engine equals lower pressure drop across the engine and thus lower air velocity across the engine and thus less cooling.
If you can make your conventional baffles as tight as a good plenum system, the result should be equally effective. Extreme attention to detail including allowance for all that engine vibration and relative movement relative to the cowl must be taken into account. I don't know how to do this. Others may.
There is another way. I saw a photo of one racing aircraft that used rubberized fabric above the engine attached as rubber baffling would be around the perimeter of the top of the engine, but forming an complete "balloon" or tent with some kind of coupling to the cowl inlets. When pressurized by inlet flow, it blew up and was resisted by the upper cowl, same pressure and loading as with conventional baffling. But no baffling gaps from poor installation or vibration. Think flexible, inflatable plenum. Unconventional, but should be as effective as a well designed rigid plenum as it creates maximum possible volume over the engine with absolute minimum leakage, IF it is well executed.
"If a plenum is not used, the best method for measuring the seal of a typical cowl would be differential pressure measurements from pitot at inlet to static inside the cowl?" Er, um.... Short answer: No. Let's be careful here. Which static inside the cowl? If you measure ship's pitot to static above cowl, you get an idea of the amount of total pressure recovery your inlets are delivering above the engine. That is useful. One would like 80-90+%. But you will NOT get a measure of leakage. Other than the way described above, there is no easy way to measure leakage.
The cooling of the engine is dependent on the pressure drop between the top of the cowl and the bottom of the cowl, that is, pressure differential across the cooling fins, top to bottom. To measure this, plumb a line to above the engine, and a second to below the cowl, and put these lines on a water manometer held by your co-pilot. You can make a manometer from a wooden yardstick with a clear tube forming a U around the yard stick, held on with tape or plastic clamps screwed to the yardstick. Measure the pressure difference as the difference in water level on the two sides of the U. This value is what engine cooling depends upon.
Usually in the real world at climb - cruise conditions pressure drop from top to bottom should be 8-10 inches of water at the lower altitudes (say 6000-8000 feet), more (possibly much more) at high altitudes (teen's and twenties) due to thin air effects. If you are not getting the required pressure drop across the engine, then your CHTs (and probably oil temperature) are probably high. More power requires more pressure drop to carry away the heat.
If you measure ship's STATIC to bottom of cowl, that gives the pressure drop from inside the bottom cowl through the exhaust ports back out to the ambient. If it is high it could be because the exhaust port area is too small (unlikely in a Lancair IV, probably same in a Legacy), OR it could be because you have huge leakage around the engine due to poor baffling.
Check numbers.
1) You should have total pressure from ship's pitot tube to above the cowl to see how well the inlets are working.
2) You should have pressure drop across the engine (top to bottom) to see what actual engine pressure drop is.
3) You should have lower cowl to ship's static to get pressure drop going out the cowl exhausts.
4) All three numbers should sum up, within experimental error, to equal total pitot tube pressure which will be expressed as IAS. If not, try again.
Good data is hard to collect.
I hope this helps.
Fred