Great post. Thanks.
I think the important part is all things being equal. The few divers I've seen in full foot fins are wearing teeny tiny snorkeling fins that look like they came from walmart, and those aren't very efficient; the divers look like they have to work hard to get anywhere if there is no current. So a blanket statement that full foot is better than open heel doesn't really work- because you need to compare GOOD full foot fins to GOOD open heel fins.
But I absolutely agree with why a high quality full foot fin is better, if propulsion in the water is your only concern. (Being able to walk around comfortably is a huge benefit for me of open heel.)