With the help of my buddy, I got my GT500 mufflers on my 2013 GT last night. The 4" resonator tips fill up the exhaust slots in the rear valence very nicely, and the mufflers have a better, throatier tone.
We lifted the car onto four jacks. With that wiggle room, it would have been impossible to install.
It took 20 minutes per muffler to get the originals off. After a 30 minute break, it took 2 hours to install the GT500s. I'm no gear head, and I'm certainly not a grease monkey, so I was frankly out of my league. My buddy has worked on cars, but this was the first time he worked on a Mustang. To install, we ultimately found it helpful to use the jack up the mid pipe to provide enough clearance on both sides, and on the drivers side we had to drop the panhard bar to provide enough clearance. These were key to the install.
The quads would be cool! Will they fit without modifying the rear valence? This 4" tip fills it nicely I must say. The throatier tone is pitch perfect.
The Ford Racing GT500 axle back kit with 4" tips is 50 state legal.
If you aren't worried about legality, the cat-back systems that you get will be louder as well as lighter, giving you a sportier tone but also reduced weight in the rear.
GT500 axlebacks are great on the GT's. I did that to mine in 2011. It gives it a nice bark without being overbearing or droning. If you are looking for a mild upgrade, they are definitely the way to go, particularly if you are on a budget.
Thanks for the input. I figure they are both street legal. I'm looking for a good bark but not a terrible highway drone. From videos I like the soulnd of the Roush and GT500 and havent heard of any drone with either one.