I just recently installed a Magnaflow Cat Back exhaust (part #16630) on my 2008 Hummer H3 3.7L I5 with 116,000 miles. Most H3 Magnaflow videos I see are for the 5.3L V8 Alphas and from all the research I've done, there is definitely a difference in the sound between the two. Be mindful that this video was shot with my iPhone 11 and the sound really does not compare to what you would hear standing next to the car.
My thoughts after 200 miles:
I love the low growl that you get from the Magnaflow. It makes the I5 sound a lot more aggressive than the stock system. Really looks great behind the car, too. I'm not really sure I can feel a substantial difference in power or acceleration but each vehicle will be different. I already have the Air Doc intake and a performance tune from PCMofNC so the addition of the cat back exhaust might not be as extreme if I didn't have those. Overall, I am really happy that I finally did this mod after thinking about it for years.
There is a lot of discussion out there regarding "drone" and my recommendation would be to take each comment with a grain of salt. Drone is very much objective. Some don't mind the deep tone of the exhaust while others have trashed the entire system because of the change in sound while driving. Not really sure what some people were expecting when they made the switch or if they hoped adding a Magnaflow exhaust would make the H3 sound like a Tesla. It's definitely a deeper tone and I notice it more while driving at slower speeds around town than on the highway. I didn't want something that was going to be obnoxiously loud because my 1 year old son rides in the back on a daily basis. Some comments on the back seat noise level made me hesitant to pull the trigger but I just don't think it's that bad.
Hopefully this video helps someone get a better idea of what to expect when making the switch. I rely so much on videos like these and hummerforums.com when working on my H3 so I wanted to do my part to help someone else that might be wondering the same things as I was.