Did you change the corresponding value in "Edit Other Levels" as well (if you're on auto brightness)? Not on AOKP, but mine is fully functional both on manual, and auto.
Their implementation is the same as CM9 (which is nearly identical, if not the same as the CM7 implementation).
Edit: just a thought, not sure if the AOKP status bar brightness slider is hardcoded to a certain minimum level? Are the results the same if you adjust your brightness via the Settings>Display>Brightness slider?
Hammerhead / Flo / Mako / Maguro / Odroid U2