I made mod with lower values of brightness when we set it to autobrightness. You can check it by going in dark place and pull brightness bar to its minimum. Then tap on autobrightness, it would be darker. Now it is the same, minimum and auto on dark room. Its not just lower values changed, others are too. So it could get little more battery life (only if you do not use minimum brightness all the time offcourse).
It is CWM flashable. I know it is working on XXLQ4 JB firmware, didn't try on any others. If you want to try with non XXLQ4 version of JB, please do a backup before. You can just backup framework-res.apk and replace it with the on in CWM zip, after that you can flash it from CWM and restore to previous state.
If someone want more darker, let me know.
34 101 168 204 255
2 15 23 100 200
1 5 15 90 180