Just weighing in on this badboy (awesome thread, gutted I didn’t find it sooner).
One major improvement I’d like to see to lightmass is an improvement to the map processing. Currently it’s not possible to divide a single texture up across multiple threads, so build times are always capped at the largest lightmap resolution you have in the scene (usually anyway), and only one thread can do that work while the rest are doing nothing. Something similar to xNormals method where lightmaps are processed one-by-one and split accross all the threads might make lightmass significantly faster. I would have a pop at it myself if I had the first clue where to start, but this is probably one for the rendering team anyway.