I checked this again a few days ago with my RME Fireface 400. I did a reamping loop and recorded spdif and main out simultaneously. At 44.1 kHz, spdif arrives around 1 ms later than analog. Interestingly (and that is why I was measuring), at 96 kHz, spdif is half a ms later still that at 44.1 kHz (1.5 ms later than analog in total). When I measured a few months ago, I measured higher additional latencies with spdif at 44.1 kHz, around 2-3 ms. I did not investigate wether this has to do anything with the specific patch (I used the same patch in the latest comparison, but a different one in the first).
It would be interesting if tech support has anything to offer on this subject!
It is defintely related to the audio interface, not the Kemper. On my RME HDSP 9632 , SPDIF arrives a bit earlier than analog.