I'm rendering the full almost 6 min video and will post it here. I only showed the last 43 seconds and did a screengrab of the airspeed indicator and magnified it for easier verification.
Phil, I have no reason to BS. It is what it is.
EDIT: Here is the full video from which I took my first video. The first minute or so is level flight at reduced engine settings, then I increase to 3000 rpms/18 lbs boost. I'm aware of the up and down porpoising, but even on the upward stroke the speed does not decrease below 330 mph IAS.
I did another run just now testing the radiator at auto, then open. Whenever I opened the radiator the speed would slowly drop below 320 mph (and radiator temp rapidly drop), when selecting auto the speed would gradually increase above 330 mph IAS and rad temp would rise to 110 and hold.
I tried adjusting elevator trim to 1 degree up per your video, but that made no appreciable difference in IAS that I could detect.