Yo I installed the bullet dymax muffler and my car sounds much better. How much horses added who knows!!! But read the following.
I did the Free Air Ram Mod two weeks ago according to www.installuniversity.com and supposedly after doing this I may experience a slight "whistling" type sound from time to time. Well I never heard it and I ran my car hard quite a few times. However after doing the dynomouth mod I can hear the whisper!!! I know I have a few horses added because even when I drive normal I feel the car is has a little more pep to it.

Now for you flowmaster lovers or those considering it....DON'T DO IT!!! Let me tell you why. I got my car and it had flowmasters on it and soon as I changed over to the dynomouth mod I felt the difference. If you looking to get a loud sound on your car and you don't car about horses...do it cheaper but just removing the muffler...YOUR CAR WILL BE LOUD AFTER THAT!! Why put on flowmasters that won't give you real flow!! Get some horses while you get that loud sound you want. I love the bullet muffler from dynomax cuz my car isn't loud at idle, but when I rev her up you can hear the growl!!!

Parts and labor $180 @Miami Muffler