I saw on internet lots of nozzles of very little aperture…0,3 and 0,2 mm too…
Its possible have a improving of quality of x, y layers upgrading with that little hole nozzles?
You could try it but usually a switch of nozzle diameters requires tweaking parameters we don't have access to. Also going from .4mm to .3mm almost halves the nozzle extruding area (area scales with the square of diameter * pi/4) so it's a pretty big difference in-terms of getting material to move thru the nozzle and you may run into problems with flow. You might want to try Julias z-temp (disclaimer - immediate void of warranty), as it allows you to increase the nozzle temp so the extrusion may flow thru the reduced nozzle area more easily.