Does the ISO setting impact this sensitivity at all? Also, there is an option to simulate the actual exposure in live view mode. If you use live view + a long exposure, does it intensify the output somewhat? I just did some very preliminary testing of these settings, and found that ISO did not seem to make a difference, and that exposure times longer than about 1/10 of a second were not adding anything to the LCD brightness. It would seem that the default setting of automatically adjusting the LCD brightness may in fact turn out to be the most sensitive.
It would be worth checking these settings on something more appropriate than my wall, though
