There are several problems with using studies to measure change in economy or efficiency in running form. Not to pick on Jtupper, but in a study done like the one he mentioned several things effect the results. First off, ask any good mechanics coach and they'll tell you that running ona treadmill alters a runners form. It's harder to do certain things on a treadmill that are natural running outside. Using myself as an example when I run on a treadmill I don't extend my hip (I cut the stride short), and have to conciously think about putting my feet down flat underneath me because on the treadmill they sometimes tend to float out on me. In fact, I've had coaches who were able to tell me when they saw me running later in the afternoon if my morning workout had been on the treadmill because in the afternoon the effects still linger a little.
Secondly, you can't just tell someone to land forefoot or heel strike and to change in the middle of a run. It takes many months or even years before someone can switch from heel striking to landing flat footed. Of course if you tell an athlete who has heel striked for years to try running on his forefoot and measure the economy it will be worst for the motion that is unnatural at first. It will be awkward because he's not used to running that way and more than likely isn't doing it entirely correctly.
Just a couple things to point out as to why scientific studies aren't a definitive answer on this subject. It's too hard to study because of the above reasons.