I've never actually used this feature (my a570 has a separate button for exposure compensation), so I may be mistaken...but if I'm not, this Fast Ev feature works by writing to the Ev compensation propcase.
Canon user interface lets you modify it in range -2 to +2 Ev, but it will actually work in range -4 to +4 Ev when altered by CHDK (unless this has changed in more recent cameras).
If you force ND state to using CHDK, Canon's autoexposure will not become aware of this. Therefore it usually only makes sense to use ND filter override when you also manually set ISO and Tv (from Canon user interface or using CHDK overrides).
