Okay ty for the info. I have delved some in the shade-c sun/shadow scripts, not yet found the problem/solution but did notice I gained a minor boost to fps without noticeable loss by setting splits to 3 (lower disables sun shadows):
sc_screen_default.settings.lights.sunPssmSplits = 3;

I also noticed that the problem only occurs for sun shadows, and not for shadows of shade-c spotlights. (Though shadows of spotlights only work well at small radius's & arcs it seems so its more supporting it than replacing sun entirely.)