Gentlemen, I know what I am talking about, GIFillIntensity 0.3 never will get that disaster of a lightmap, the interiors will be a tad dark (but playable though) but in any case the exteriors never will be so dramatically dark like in your pic.
I use 0.4 in all my maps and I must write it each time I open the editor, I never tried to write the line manually in the files inside map/editor folder though, maybe this could store it