Helped a friend install his MS CAI today.
(I have the Cobb SRI)
Here are my impressions...
Sound:
Cobb is a just a little bit louder. You can hear the turbo whistle a bit more, if that makes sense. The MS CAI sounds just a little muffled in comparison. But really it is close to being the same.
Performance:
I installed my Cobb SRI last week, and have already gotten use to the increase in power. I drove my friends un-modded MS3 before we installed the CAI, and it felt anemic. After the install, we drove around and did a couple freeway pulls from 40-90mph in 4th gear just to see any differences. My car pulled maybe 3/4's of a car length consistantly over the 3 times we did it. I chalked it up to the fact that my ECU has had some time to adjust to it. Overall, I after driving his car, they are pretty equal in performance.
Really there is more of a difference in sound (albeit still quite minimal) than there is in any performance feeling IMO.