If you own a 500D, try these steps:
Go to a location with low light
Activate the movie mode
In the Expo menu, make sure "Exposure override" is deactivated
Increase the "LV display gain" setting to, say, +4
Record a short clip
Now reduce the "LV display gain" back to zero
Activate "Exposure override", and maximize the exposure (shutter, aperture, ISO)
Try increasing the "LV display gain". You should not see any effect past +1, if even that
Record a short clip again, and compare it with the previous clip
If you're seeing what I'm seeing, the latter clip recorded with maxed manual controls should be much darker, and the clip recorded with working LV display gain much brighter, although much noisier as well, of course.
However, I'm setting the priority of this issue to only minor. My reasoning is that the "LV display gain" setting is only used in low light shooting, so the deactivated "exposure override" setting should be fine because the camera is likely to select maximum ISO and aperture opening anyway (and the shutter speed can be controlled with the FPS override function).
PS: Thank you very much for both settings. :)