Doing what you suggest and keeping the layer height to < 80% nozzle diameter would get you to 0.6mm, which would be great.
I understand going beyond that is going to push the melt speed to the limits to keep the flow rates up. I believe this is why they incorporated a longer melt zone on the Volcano, allowing the filament to pick up more heat.
0.9mm layer height should be possible! Fairly chunky looking I would think.
I have already ordered the knock off Volcano kit from China. I’ll let you know how it goes.