Usually you’d set your first layer thickness to at least 0.2mm to compensate for a somewhat uneven bed surface. Getting it trammed/leveled/squared to 10 microns or better (which is the ballpark you’d need for a 20µm layer to stick) is absolutely impossible imo.
I don’t have a Ultimaker 2 but would happily use it, if @Ultimaker wanted to send me one.
20 microns should be easy enough, though. The trick would be to set the 1st-layer height to a more reasonable number and let that act as a level (or print a raft).
This is similar to the approach I used to print < 1 micron layers on my Ultimaker 1. The hard part isn’t getting an Ultimaker to move at these types of resolution - it’s getting it happy enough to move really fast, which you need to keep the extruder moving, and waiting forever for a print to finish…
If you’re printing down under 0.025mm, your machine is probably in fairly decent shape.
Extruder speed isn’t really a huge problem unless you have layers with small area - if you have those, you’ll need to disable min-layer-time or balance it vs the min usable feed rate that keeps the extruder moving.
I don’t have it set. 5-8 weeks of waiting.
However I know that I’ll print a lot of important things as high resolution as possible to increase the level of details visible and reduce the sanding required for a perfect surface.