Is that an assumption, or your actual experience? I only print functional parts, and I have no quality or dimensional issues printing at 0,8mm line width and 0,32mm layer height with the stock 0,4mm nozzle. Then again, I consider +/- 0,1 mm accuracy reasonable for 3D printed parts.
I think for a lot of parts, printing at 200% is an overlooked possibility. It’s very easy to try, just by changing a few slicer parameters. Whereas changing the nozzle is more involved and also requires to change those parameters.
The formula is Volumetric flow [mm³/s] = line width [mm] x layer height [mm] x print speed [mm/s]
Hence: Print speed [mm/s] = Volumetric flow [mm³/s] / ( line width [mm] x layer height [mm] )