If you change the extrusion width from 0,4mm to 0,15mm without changing the nozzle you will run into all sorts of troubles !!
Your printer needs to know exactly what nozzle he’s working with and the actual nozzle size the printer reffers to is usually 1,2x the one of the nozzle bore diameter. The printer interpretates the nozzle as being a bit bigger hence when printing the printed line will (due to gravitiy and the force of extruding downwards) be about 1,2x thicker then the nozzle size.
If you select such a small extrusion width the printers nozzle will run around the same area multiple times to fill in the hole as it thinks that it’s working with a smaller nozzle.
I would try to buy smaller nozzles and additionally you should change the 3d models letter size to at least 0,4mm while 0,8mm and thicker would be much better.
If thicker letters don’t fulfill your need you might want to look into engraving machines or services.