Sky-lit sectors should not be dimmer than non-sky-lit sectors with equivalent light level
The sky is supposed to be a bright light source, so it should not subtract light from a sector, only add to it.
Could be worthwhile to investigate non-saturating over-bright light levels (255+) that behave as HDR light values. The renderer could dynamically adjust the exposure depending on which sector the player is looking at, so that over-bright sectors would be the fullbright ones and others would become dimmer.
Good test map: AV map23. The red sky makes sky-lit sectors too dim.