I have a 4" Heartthrob Cat-back exhaust and love it. I wen't with it after I saw a Gearz (. . . I think) episode where they were talking about quick mods to ecoboosts (you can find it on youtube). It does make it a little louder inside the cab, but I've gotten use to it and barely notice it when the radio is on. It is much meatier sounding outside the cab, but that's what I was going for - I like the meatier sound. You can also really hear the turbos spooling - which I think is cool. Haven't really noticed any performance increase, but wasn't expecting much with just an exhaust upgrade.
I watched the same video that MattGeo watched and I also have the Heartthrob exhaust on my 2011 Ecoboost. One thing I'll tell you is that if that's the only thing you do, you're going to have an MPG drop. Slow tells me I need to get a tuner to fix that issue and I'm waiting on my tuner to come in. Once it's in, I'll be sure to let you know if it doesn improve my mileage along with the exhaust. The exhaust itself is great, I love the meatier sound and I can hear the turbo spooling better than I could with the factory exhaust (which I love).