1) The research article was published about a year ago :
A Comparison of the Energetic Cost of Running in Marathon Racing Shoes
by Hookgamer, Kipp, ...
https://link.springer.com/...07/s40279-017-0811-2


2) I own the VaporFly 4% for nearly a year also, and yes it is efficient, especially for heel striker (the study show it bring more than 4% to heel striker, and less to others), but it is also very unstable at heel. Too unstable for me.
So, for me, and for many others, this shoes does not deliver the 4% improvement.

For me, it probably deliver 2% for 10km (because I mid foot strike), but is useless for semi and longer.