I’ve been doing some more thinking on this, and I now think it should really be a low priority.
First, we have to assume that the user has equipment that can understand 7.1. If they have a really old AVR that literally can’t conceive of more than 5.1 channels, then that’s a big issue, but the number of people with a setup like that and needing a device like the Vero 4K for the video end is very small, so I think we can ignore them.
For standard 5.1 (L+R+C+LFE+LS+RS) PCM (or PCM-out audio like FLAC or AAC) in the source video, there are effectively 3 cases in the user playback equipment:
Much less than 5.1 speaker setup (i.e., not just missing a subwoofer). OSMC shouldn’t care about this, because such setups should have the Vero 4K set to 2-channel output, or should let the equipment deal with the fairly complicated re-mapping of channels.
7.1 or greater speaker setup. Sending 5.1 as 7.1 with silent LSR+RSR will only affect the ability of technologies like ProLogic IIx from expanding the content to get sound from all speakers. No sound from the mix on the source will be lost…there will just be silent rear (and possibly 3-D) speakers. It would be the same as listening with just a 5.1 speaker setup.
5.1 speaker setup. I think this is actually the biggest issue, as sound will be lost, since the downmix from 7.1 source to 5.1 speakers will mix LSS+LSR source to output to LS speaker. Since LSR will always be silent, this will mean that the LS output will be too quiet on movies with 5.1 channels, and OK on movies with actual content in the rear speakers.
But, I again don’t see a lot of people who want a device like the Vero 4K only having 5.1 setups. Sure, you could see much lower configs, like soundbars in bedrooms, but anybody with a Vero 4K and at least 5.1 speakers probably has the full 7.1 (or greater) setup. So, almost nobody will lose anything of significance with the current way sound is being output.
So, my vote again is to put this into a “nice to have” priority, but definitely things like HDR->SDR tweaks should be more important.