We are pretty close already, at least in z. Out of curiosity I ran a small, rudimentary test series recently. Turns out the real limit for FDM is minimal flow controllabity.
0.02mm3/s were still doable. I feel very confident about a 0.03mm layer height with a .4 nozzle on an X1. Lower is possible, but it does get tricky.
Interestingly, at such low layer heights, a smaller nozzle makes us hit the flow control limit sooner.
It would be interesting to see if the improved extruder motor of the H2D enables better flow control at such low flows.