Have Borla ATAK on my wife's Mustang and it is great. Don't know if Mustangs gain power from axle-backs though. Usually Cat-backs will widen the pipes to 2.5 inches from the stock 2.25 inches, a little bit higher flow.
Wife = 2012 Mustang V6 Premium Grabber Blue BBK CAI, BBK LT w/ HFC, Borla ATAK, MPT Tune, DSS Driveshaft, 3.73 FRPP Gears, BMR LCA's | Best = 13.5 @ 105
Husband = 2010 Camaro V6 Synergy Green Edition IPF Supercharger Best = 12.5 @ 111
Closest finish @ the line = Wife by 0.0028 seconds (4.79 inches)
Also own a 2014 Mustang GT Premium, Brembo Package, 401A, Comfort Package, Shaker Pro System, Pedestal Spoiler