Well I have been running CM6 nightly builds lately and cant say battery is much worse than any other rom for me.. but I am overclocked with screen brightness turned up so guess I am so used to being plugged all the time anyways that low battery doesn't phase me..
you can try tonights nightly build and see if it helps I am sure it has some improvements (at least a newer kernal version) over the RC..
http://mirror.kanged.net/cm/nightly/supersonic/