I know this is an old thread, but I stumbled into it looking for something else. I recently did some calibration work on my projector (JVC HD350) and I compared the peak output (using a Tecpel 531 lux meter) for different contrast settings:
At '0' the flashing bars upto about 250 were clearly visible, '+6' brought it down to 240 and '+9' clipped anything above 235 in that I could see 235 flashing but not 236.
FWIW these were the lux measurements at the screen with a 100 IRE pattern:
Contrast 0, 117 lux.
Contrast 6, 133.5 lux.
Contrast 9, 135.4 lux.
I don't think this difference would be enough to really notice unless you had two identical settings on the projector that you could flick between. In that if the '0' contrast setting was too dim then I don't think the +9 one would save the day so to speak.
Sorry to dig an old thread up, but it seemed useful information.