I have run tests on a 788T and have found the LEDs draw very little power so whether or not they are on has little effect on battery run time.
post your results, b/c if you run them bright, they draw a decent amount of power. I have experienced this with the 744.
OK, here is some data:
Test set-up is a 788T, powered by external battery only, all inputs turned off, not recording.
LEDs and backlight OFF = 0.65 A x 12.9 V = 8.4 watts
LEDs and backlight ON dim = 0.72 A x 12.8 V = 9.2 watts
LEDs and backlight ON bright = 0.90 A x 12.8 V = 11.5 watts
So it will cost about 0.8 watts to run the lights dim, and 3.1 watts to run them bright. I agree running the lights bright takes quite a bit of power. The last time I tested this I only checked the power draw with the lights on dim since I record indoors 95% of the time and the dim lights are bright enough indoors.