And since switching to Hoka, Walmsley has lowered his previous CRs. So, at the very least, we can say those clunky shoes haven't sapped that much of his energy, wouldn't you agree?
There's no data on efficiency re: trail running shoes. Also, what's efficiency? Running economy? Energy return? One thing you can say about Hokas and other maximally stacked shoes is that they reduce the amount of pounding, which sure provides some advantage on longer races.
Maybe it would interest you to know that Francois won the race wearing maximally stacked Salomon prototypes - and not only maximally stacked, but also plated!
Ludovic Pommeret won UTMB wearing Hokas, and the 2nd finished that year also wore Hokas. Same for Caroline Chaverot. A number of runners have done well wearing Hokas and other maximal shoes in the UTMB races and similar European mountain ultras.
Bottom line: IT'S NOT THE SHOES.
PS: I'm not into Hokas, my current favorite running shoes are the Adidas Boston 6, New Balance 890v6, Brooks Launch 6 and Saucony Freedom ISO (v1).