I don't have the hexpander but I'm about to hook up the piezo side on my Barker. The Hexpander attaches onto that. So I'll be able to report on part of that system fairly soon.
FWIW - I've used the RMC setup and it works reasonably well down to say A - though you have to be clean. It worked way better than a GK3B... Below that latency with the current technology is an issue. My answer was to transpose my patches and octave down and play them an octave up. Not super convenient but it was workable. I was driving a GR-20.