Look, I'm a huge fan of most of the work the science of sports guys do, but in this case it is obvious that they are endurance athletes and that they aren't biomechanists.
I also hate the POSEr method, but what do you expect in such a short term study? Of course there economy will get worse when changing technique initially.
Your changing neural programing that has taken place over years of running. Go try and change how you hold your pen when you write...you will suck at it. Even if you write incorrectly, like I do, you'll be useless at the most efficient and correct way of holding your pen while writing. Why? Neural programming.
Onto the other study, did you read it?
Ground Contact Time was found to be less across all speeds with the midfoot compared to rearfoot group. That is significant, because as runners we are always trying to find ways to minimize GC. See CHapman's presentations at the recent USATF clinics for more info.
Secondly, and perhaps MOST importantly.
This study was done in Japan. Japanese are famous for their mileage run. Due to this, and other factors, Japanese elites tend to be heel strikers. It would be interesting to see how many of the top runners were kenyans who heel striked and how many were japanese.
Also, at 15k in a half marathon, you'd expect that as the runners tend to tire, some will result in a shift to more of a heel strike due to fatigue.