Hmmm...installed the 'dry' version, and really didn't feel or hear a difference. Of course, you aren't going to really feel 10HP and I have the Eisenmann Race on, so I can't hear squat other than that anyway.
Worth it? If you wanna know your flowing more tahn stock sure. Combine this with the RPI scoops and it meets/beats the Gruppe-M in the $:HP ratio (it loses in the bling battle though).
Anyway, my 2 cents...