I think the only reason you would be seeing CUDA usage reducing your total power consumption is due to the CPU you are using. As a general rule CUDA implementation is less efficient than Quicksync CPU, so if you were using 20 cams on a 6th gen i7, probably you would not see this same impact.
However, while in your case it might very well make sense (or for others using an old Xeon) non-Quicksync cpu, you could also significantly reduce your overall power usage by just dropping $300 on a refurb 6th-7th gen setup. Going that route the system would pay for itself just based on electrical usage.