This [1] is from 2013, so I don't know how it holds up, but I found it in the criticism section of the Wikipedia article for webp [2].
edit: This [3] is a follow-up from 2014 to the article/study from 2014.
TL;DR: "We consider this study to be inconclusive when it comes to the question of whether WebP and/or JPEG XR outperform JPEG by any significant margin. We are not rejecting the possibility of including support for any format in this study on the basis of the study’s results. We will continue to evaluate the formats by other means and will take any feedback we receive from these results into account."
edit: This [3] is a follow-up from 2014 to the article/study from 2014.
TL;DR: "We consider this study to be inconclusive when it comes to the question of whether WebP and/or JPEG XR outperform JPEG by any significant margin. We are not rejecting the possibility of including support for any format in this study on the basis of the study’s results. We will continue to evaluate the formats by other means and will take any feedback we receive from these results into account."
[1]https://blog.mozilla.org/research/2013/10/17/studying-lossy-...
[2]https://en.wikipedia.org/wiki/WebP#Criticism
[3]https://blog.mozilla.org/research/2014/07/15/mozilla-advance...