This is an old thread, but what the heck, I'll join in. I agree that you will never get a 4cyl to sound like a V8. You can make it louder and raspier, but deep and aggressive ain't going to happen. The big thing you have to also consider is the exhaust system drone. Nearly every 4 cyl exhaust delivers some drone while at cruising speed.
Another thing to remember is that exhaust sound is very subjective from individual to individual.
I'll probably go with the Rev9 which is reported to be pretty darn quiet under power, yet deeper and more throaty at an idle. It does get rid of the resonator thus dropping the weight of the exhaust by what I have been told is about 40 pounds. The road block I keep running into when I go to order the Rev9 exhaust is that it cost the same as a payment on my Mustang. When I go to hit the "Buy" key on Amazon, I end up going over to the payment link on my Mustang and making another payment. Down side being that I still don't have a nice free flowing quiet exhaust, Upside being that I am way ahead on my car payments.
One last thing to consider, are you looking for a sound that others outside of your car can hear, or a sound of throatiness that you can hear? If it is for others to hear don't waste your money trying to impress others...…...it ain't going to happen. If this is for your ears that desire the sound of impressive Ecoboost power as music to your ears, then buy the Ford Performance Calibration Pkg. After you install the tune and intake (which is the same intake as on the Mustang 350) the piped in exhaust sound of the stock Mustang will be deleted. However, the deep growl of the intake as it sucks in massive amounts of air is beautiful music all of its own. The extra power of the FPCP is a bonus for less than the price of most aftermarket exhaust systems.