The good news is that you can find that out right now. DJI HD FPV has been available for over a year, this is just their first drone offering in the category.
The short answer is that it degrades very gracefully. They have what they call 'focus mode', where the edges of the video lose quality before the middle (where the data matters the most). Once you've used up that signal degradation threshold the rest of the frame slowly degrades to a lower bitrate, and then after that latency starts rising.
After flying analog for 5 years it only took me about 1 day to strongly prefer the DJI FPV video solution. The range is far better, and the video is mind-blowingly clear when compared to analog. On anything but my racing specific drones I have been installing the DJI HD FPV system instead of normal analog gear.
