I am using a single directional light with a shadow with my game. After much fiddling, the best settings I could get were by using the PSSM 4 split mode.
If you look in the project settings > Rasterizer you can select the "debug shadow maps" option which will show you in the editor/game how the shadow is being laid out. This will give you an idea of the difference between , for instance, the PSSM4 split and PSSM2 split modes.
In the project settings >Rasterizer options you can adjust the shadow map size (you may need to go up to 2048px) as well as the filter type. I am using the PCF5 filter and a 2048 map and getting quite good results for my topdown shooter. AFAIK the Esm multiplier and Blur passes only effect the shadow if you have selected the Esm filter in the project settings. They won't do anything for the PCF modes.
There is a fairly major hit to using just one shadow when using my Intel integrated card on my laptop(a HD4000), it goes from 200fps to <60fps. When using the mobile Nvidia card on the same laptop, the fps is much better and it never really goes under 60fps. The Open GL drivers on these integrated GPU's are pretty iffy, so not much you can do here. There will be a refresh to GLES3 for Godot later this year, so hopefully we get much better shadow quality and performance with it!