Let's say I am 200lbs and deadlift 400lbs for reps. If I cut weight and go down to 150lbs, I will not be able to do as many reps, so each rep will take more effort on my part.
Is taxing the CNS a function of absolute weight of the lift or how much effort I am putting into lifting that weight, which will be more if I lose muscle mass?
It's a function of both, but a rep-max will tax you more the stronger you are and the bigger you are. But whether a 5RM at 400 at will tax your CNS more than say, a 1RM at 150, I'm not sure, don't think the difference is that significant. A confounding factor here is that a 5RM deadlift will probably leave you a little bit more destroyed next day because it's 5 times the volume, but CNS-fatigue is not as immediately apparent as how you "feel" the next day.
Wouldn't surprise me if there's some dweeb around here who has correlated how much his tap test has gone down after a cycling of deadlift rep ranges...