the Kinetix plenum IS noisy. both extra induction noise but I also remember a "plasticky" type noise and vibration coming from it. I didn't like that.
Also, make sure the strut bar doesn't vibrate against the bonnet, as I had to adjust it quite a bit to make it clear the Plenum.
WRT gains, I did certantly feel the difference, and when I took it off, again I felt the difference (ie less hp) and again when I put my spacer on, I certaintly feel it.