In KDE4, I modified /usr/share/kwin/effects/kwin4_effect_fade/contents/code/main.js to match my own needs, with KDE/Plasma 5, I can't find the analogy to this. (The file is still there, but editing it has no effect.)
Does anyone know where the kwin_x11 effects are set now? Thanks.

Did you try looking in ~/.config/ or in ~/.local/share ?

That's still the proper path - i'd rather say that your changes do either no longer fit or you don't effectively reload the file.
=> what changes do you apply (and what are you trying to do)?