Oh that's a good way to test, didn't think of that - although, with Telly, you don't actually need to use ffmpeg for Sat>IP - or I didn't.
I've gone back to TVheadend first for now, and exporting the m3u to Channels, I built a hugely stripped down version of TVheadend, which (I'm not entirely sure why) has more or less removed the 'bursts' of uncorrected errors that I had been seeing recently (along with fiddling with CPU frequency scaling).
Came across something on the MythTV site, that seems to offer some hope/area for further investigation:
https://code.mythtv.org/trac/ticket/13121#comment:47
"inefficient/slow due to buffering all received input data"
"Next step is that the SatIP implementation will be restructured along the lines of how the HDHomeRun support is implemented. This is a comparable device, with four independent tuners and use of UDP packets for the video streaming. The HDHomeRun works OK with only a 1Mbyte UDP input buffer (instead of 8Mbyte) and it does not do extra buffering at the input. The current SatIP implementation is largely inspired by how it is done for Ceton and IPTV."