>More to the point, even with large buffers I can't get JACK to work properly _at all_ without RT scheduling privileges
What does 'at all' mean? Like it won't even start? Or it runs but suffers constant underruns? Presumably not the latter since larger buffers should eliminate that problem. Or maybe you mean that without RT you occasionally get such tremendous latency spikes that the buffer size would need to be absurd to avoid them.
Just interested really since the behaviour you see doesn't seem intuitive.