Looks more like Q3 settings rather than lightmap issue.
Check your Q3 game settings, particularly if you have it "optimized" for online play. You can restore your settings to default (backup/delete .cfg files) and then set your color bits and texture resolution to the highest settings.
As a general note to everyone else, _lightmapscale should be used sparingly and it is usually not the best solution to the problem as you would think. Most lightmap quality issues stem from luxel aliasing. The first solution is to enable lightmap antialiasing via -samples and _filter/_filterradius.