FWIW when I was testing my cowl flap installation, I tested the effects of leaving them open in cruise. I my installation I found that while it cost 5-7 its to leave it open, there was virtually no difference in CHTs in cruise, flaps open vs closed. The only difference, other than speed was a slightly audible rumbling.
Logic says the CHTs do not change because mass flow does not change. Mass flow through the fins is driven by pressure delta, upper to lower. So, if you're sincerely curious, start with a differential pressure measurement to confirm or deny that logic. Email for a how-to sheet if you like.
Just last weekend I was messing around with the upper cowl and changed one side from the standard "abrupt" ramps to a long, gradual ramp. I tried to get the classic 7 degree divergence, but the cylinder was in the way. I ended up with about a 10 degree divergence, but the transition is very gentle. Anyway, the space between the ramp and the top of the cylinder fins is pretty tight. I think a grilled cheese sandwich would fit, but certainly not a clenched fist.
I want to hold off on a definitive report until I get some more hours on the airplane, but the initial flights show a noticable improvement in temps, and unexpectedly, my misbehaving #4 and #5 have evened out.
I note that's your Rocket cowl. Pretty sure the inlets operate at a higher Vi/Vo ratio than a standard Vans cowl, so there are some differences in how things work.
If inlet velocity is high (high Vi/Vo, velocity through the inlet is a high percentage of freestream velocity), an abrupt inner lip would probably cause flow separation along that upper, inner surface, which would spoil some of the dynamic-to-static conversion. You obviously recognize this detail, which is why you added the nicely diverging ramp. Note that now velocity (and dynamic pressure, Q) may still be pretty high at the constriction over 1 and 2, so some of the Q-to-static may be taking place further aft.
A lower Vi/Vo inlet makes the upper, inner lip diverging angle less critical. The slow flow of a truly low Vi/Vo entirely eliminates the need for a diverging ramp; look at a Cessna Corvalis or Mooney Acclaim at the next fly-in. At the same time, the slower flow requires more space above 1 and 2.
I find that the CHTs, on my RV6A (o360, carb, with 10:1 pistons and a CS prop) are lower when I climb out with full power and RPM rolled back to 2400.
Sure. That's a note right from the 1930's NACA studies. Same cooling mass flow, fewer combustion events in a given time. The NACA wizards phrased it as "for a given mixture and ignition timing, cooling demand is proportional to engine internal mass flow"...a fancy way of saying proportional to RPM.
Just remember that with high CR and a very hot cylinder, running highly oversquare at high MAP puts you close to the detonation limit.