I did 1.5" drop rear only on my '08, it leveled out the stance, depending on equipment they were more jacked up than the 2011-14's. The 2015 looks much lower....
You could measure the wheel to fender gap front and rear then subtract the drop rear only to see where you'd be on you particular car.
I did not noticed any ill effects, car felt more settled better grip in the rear.
Now: 2016 Mustang GT Fastback-Magnetic Metallic, Ford Racing Touring axle-backs, Ford Racing front strut and k-brace, K&N CAI, sound tube delete, 1975 Corvette Coupe-L48, 4-speed
Past Lives: 2001 Corvette Z51 Coupe, 2012 Mustang Grabber Blue Coupe, 2008 Mustang Torch Red Vert, 1988 Mazda RX-7 Vert, 1985 Nissan 300ZX, 1979 Mazda RX-7, 1971 Datsun 240Z, 1972 Fiat 124 Vert, 1971 Chevelle, 1970 Challenger