They briefly mentioned HDR in the previous Forza article and (surprisingly) said it didn't offer that much in this title. Obviously the TV used is as important as the feature itself when it comes to HDR though. I'm sure it will offer huge advantages in many other titles.
I think for games released now the hdr is all about contrast, for movies it's contrast and a much increased colour space so the change is even more pronounced.
I assume games can target rec2020 or DCI P3 but I believe the current HDR titles are not and just bumping the contrast and brightness.