Can't say much about Scarlett - Clarett comparison, but ADAT pipeline itself will not introduce any significant latency. The theoretical statement : " Super-low roundtrip latency was measured at 2.74ms, working at 96kHz with a 32 samples buffer on Logic Pro X, running on a Mac Pro and OS 10.11" on Focusrite site is saying nothing, as Your CPU load will force You to use Signal and Vector I/O size that will allow Your patch to run smooth.