Thanks for the measurement, that’s very consistent with the actual settings. The MOD runs
jackd -R -P 80 -d alsa -d hw:MODDUO -r 48000 -p 128 -n 2 -X raw
With jack2 (async by default), the nominal round-trip latency is 128 * (1 + 2) / 48000 = 8.0 ms which matches your measurement. So there is no systemic latency.
PS. if you launch jackd -S ... or use jack1, the nominal round-trip latency would be 5.33 ms. In case of the MOD the soundcard is using i^2s/i^2c with DMA (no additional kernel buffers), the codec (hardware) latency is on the low usec scale (it’s a CS4245).
PPS. for a good explanation sync/async and periods see http://lists.linuxaudio.org/pipermail/linux-audio-user/2012-May/084743.html