Bruce, you answered your own question. Great trouble shooting.
I am not having the trouble myself but have seen a lot of people post in the forum about this problem so I think it will help a lot of them.
So it ended up being the allow external devices setting. I think my is turned on and why I probably didn't experience the delays, I was toying with these setting and thought it might be some thing like that, but thanks to your excellent trouble shooting you made a definitive answer.
My RVU isn't straight Ethernet either, it is bridged to the MoCA. I know that a lot of people like to run straight Ethernet but I never understood that, as you said the MoCA has greater bandwidth and it's design makes it really robust. Though there may be inherent latency in the bridge translation, which I was thinking may have been the culprit but from your testing it does seem just to be a setting. Besides, I have no idea how you could packet sniff or do traffic analysis on the MoCA, I suppose they must have similar tools.
Its striking to me that in your findings that 1080P content is only using 7.5 M but the user guide text spikes to 20, 30 80 Mbps.
Again, really good work.