Bear in mind that the "noise level" is not an exact science at the end of the day, and your eyes should be the final judge :) As part of moving to 1.7, for example, you could raise the noise level from 5% to 6% or even 7% and get visually the same amount of "cleanliness" in the image. There could be something there worth investigating, but ultimately if a noise level result changes from 6% to 6.5% but the image looks better and renders just as fast or faster, then your eyes are the true judge and the shift in measured noise level is just that, a shift in how it is measured and not a shift in actual image quality.