We did a more in-depth investigation into Dolby Vision performance, and found that there is a difference in brightness between the native apps, and when playing Dolby Vision content from an external player. We took brightness measurements of the same spot in a movie, and compared the measurements from the native app, from an Xbox One, and from an Apple TV. On both the X950G and X900F, the native apps were brighter than the same content played from the external devices.
As a sanity check, we then took the exact same measurements from the same equipment on the TCL R617, and the LG E8. With both of these TVs, the brightness measurements were extremely similar (within margin of error), regardless of source.