For the few people interested in the discussion about x.v.color we had a few pages ago, I have found this excellent thread
It looks like with the right hardware and the correct settings, there are some clear differences for some movies between the standard edition and the Mastered in 4K blurays. Now how much of this is due to the extended gamut gamut, and how much to the better bitrate and processing, it's difficult to know as the colour differences seem subtle, but I read the entire thread and they went through the trouble of detailing which titles were most different and in which aspect (Ghostbusters, Spider-Man and Men In Black are amongst the favourites).
Apparently the main reviewer can see a clear difference when x.v.color is enabled or not, but only with recent Sony 4K displays with Triluminos. Older Sony displays with only x.v.color compatibility or the old triluminos feature didn't seem to display any of the extended colors.
In any case, it looks like your best bet to display the extended gamut info is a recent Sony player and a recent Sony 4K display, both with the "Triluminos" feature.
A member even did some tests showing that the max values for YUV shot above 235 with the Mastered in 4K blurays
. Now whether this shows actual x.v.color information or wtw info, I'm not sure we can say, but the thread is very informative for those interested in the subject.
I suggest we comment in that thread rather than re-opening an O.T. discussion here