Yesterday evening I sat down for some minutes and improved the lightmap generator of irrEdit
with subSampling. That's just a bit like the technique known as AntiAliasing. This is the result:
The lightmap texture on both pictures has the same size (256x256, for the whole level, not per face). On the right pictures I'm calculating 4 samples, on the left only one. The speed decrease is not 4 times as expected, but a lot less, I guess because of caching, nice. I added the option to choose between 1, 4, 9, 16 and 25 samples per texel. Will be in the next release.