Can you feel 8ms in a way that distracts you?
8ms – that corresponds to 2.75m of sound in air. Most guitarists move around a lot more around on stage.
Acoustic guitar lap to ear is about half of that. There are very few musicians who notice or can’t cope with 10ms latency.
Humans are a lot more sensitive to jitter though (variations of latency). There are not many studies on the subject. One that comes to mind is http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0127902 which measured 20ms as common fluctuation for hi-hat onsets and snare/hh concurrency.
PS. It’s different for vocalists (sound via bones to ear vs AD/DA latent headphone). So if you use the MOD for vocal-processing with a headphone and without direct monitoring of the dry-path. Maybe 8ms is not good enough.
As for async/sync mode. In sync-mode the realtime-constraints are tighter. Effectively you’ll have a higher worst-case DSP usage for the same pedalboard or x-runs become more likely.