There's no doubt that much faffing is necessary with this stuff, but it's nice when it starts to pay off.
In answer to your question Pete, I would predict no difference in sound between Pi via M-DAC and CXN via M-DAC, when playing FLAC files.
It's the CXN's dual DACs and 'ATF2 24 bit, 384 kHz' proprietary upsampling that could make a real difference to its sound when compared to anything through the M-DAC. And of course, native DSD should sound better than FLAC (subject, as always, to the original recording quality)
Apart from its sound, a lot would depend on how good the CXN control app is. (Although I do like the idea of its scroll / push-enter front panel control).
Like you, I'm happy with the Pi and DAC for now - but haven't ruled out a CXN one day.....the real skill is learning to remain content with what you have.