You'll have gathered by now that I don't know the answer for sure..
What I think would happen is that high bitrate PCM/LPCM audio will pass as source and the DAC will convert to stereo, but DD, DTS, DTS-HD MA etc will be downsampled to stereo 48Hz by Q5/Q10 Pro before output by SPDIF. So for DD, DTS, DTS-HD MA you'll not get much of a quality improvement.
In general I wouldn't have thought the quality improvement of a cheap external SPDIF DAC like that would be very big vs using Q5/Q10 Pro's RCA outputs.