Hi Fedor,
I dug through the code a bit, and MaxLightsPerPixel is mainly used to limit the number of local lights that can be iterated over in the VSM Projection pass. As an example, see VirtualShadowMapProjection.usf::820. Setting MaxLightsPerPixel too low introduces artifacts since the projection pass could not query a local light contributing to shadows. You will need to ensure that you are querying a higher number of local lights by increasing MaxLightsPerPixel to get around the artifacting, as we currently do not have any other mechanisms to control this behavior. This will tie into the SMRT settings you mentioned as well. If you are not tracing rays to the correct light sources, you cannot correct your soft shadows by increasing the ray or sample count. These should be adjusted once you have a sensible value for MaxLightsPerPixel. I hope that clears things up for you, but of course, feel free to ask any further questions if you have them.