Hi. The "Denoise Amount" value, is not a quality parameter, but is a mix-value, setting the blend between a full denoised image (1) and a full "normal" image without denoising (0).
So to compare ON/OFF you'll have to set "Denoise Mode" to "Full denoising", and then change the amount value between 1 and 0 (0 = OFF, 1 = ON).
I can't give you any benchmark-numbers, but you can easily set up your own test.
Hope it helps