As @DeBoet pointed out, this feature request aims to do the same thing as this feature request, which also has the same problem:
As far as I understand the issue with pre-buffering the main file is a client’s ability to handle two streams at once, which might not be a problem on most iOS/android/windows devices, but is on smart-TVs, which often call it PiP (picture in picture). Though I don’t know why a smart-tv wouldn’t support PiP if it already supports DVR 
Reflecting on that, this might not be as big of a problem I thought it would be, thinking about appleTV (which I don’t know much about but can imagine supporting PiP) and other external players to hookup to a tv/beamer.
This then would be a case-by-case situation support thing, which would prompt a “is it worth implementing for part of our customer base?” question.
Hence, I’d rather opt for a “mask buffering” option, which I know wouldn’t change the wait-time between preroll and movie, but at least make the experience more enjoyable.
I mean, even big cinema has time for dramatic pauses before the movie actually starts. Could be an opportunity for home-automation ppl to implement “dim-lights when preroll done” 
imo, the mask-buffering option is easier to implement and is easily supported across the board. so, I ask you to take a moment and maybe put a vote in on the other request as well, for chances it being implemented are higher than this request.
I voted for both requests.
