I've stayed out of all these discussions, but you've mentioned actual data a few times and I'm curious - where is this data? Did the Devs put out a bunch of info somewhere?
Publicly, there's at least one livestream where they talk about playtesters liking actual healing over temp HP far more w/r/t Second Wind; might be the same video that some folks are freaking out about. Alas, can't find it off the top of my head.
On a more basic note; ccooke's post pretty much nails it on the head; considering they had it originally as temp HP, logic would indicate that they got enough reaction from folks wanting it to be actual HP that they switched it.