Well, most people on this forum agree that column staggered split keyboards with significant tenting angle are best for ergonomics. That is also indicated by the last of the 3 studies you posted. So overall keyboard shape is the most significant factor for ergonomics (so that hands are in a natural position).
Speed will depend especially on the distance travelled by fingers. That means that chording totally wins. Then it will depend mostly on the layout (e.g. dvorak versus qwerty versus many other less know layouts). The least significant will be the keyboard shape itself.
There was a link to a study in this forum about slope and tenting (opening angle):
https://geekhack.org/index.php?topic=98277.msg2682219#msg2682219