Maybe I can give you some advice, I'm also a prior 996 owner, that steep into a '11 Mustang GT w/. The sport Brembo package, standard transmission and 3.73 gear. I did the change after I readed on MT how this car went head to head with a car costing almost double the BMW M3.
-Mustang's have very strong 5.0 engines
-steering is great
-std. Shifter is very acceptable
-brakes are strong
-you will like the street & road handling
-much lower maintenance cost
-you can personalize it to your own way, very good aftermarket
-More comfortable than your 911. People can really seat in the Mustang backseats, compared to the fake rear 911 seats.
-Handling at the circuit. It will not match the 911's at the track. Mustangs are very heavy while taking the curves, the stock brakes, even the Brembos will go away, if you are driving hard at the track.
-You will notice at very high speed the limited aerodinamics of the Mustang compared to the smoth design of 911's.
-You will miss the exotic looks of your 911. Less people will glance at your car.
Recently I test drove an automatic 3.55 geared '15 Mustang GT
Very strong engine/transmission/gear setup. I Did not had a chance to try it on any curves, I was not able to differentiate the rear suspension upgrade. I personally prefeer the prior muscle car look of the '11-'14 edition.
Probably my next car will be a 1LE or ZL1 Camaro present model or maybe the new designed Camaro.
I don't complain about my Mustang, but is time to change.
2011 Mustang GT Premium, Brembo, 3.73 & Std. transmission, Steeda Sport springs & KONI STRT. GT500 strut mounts, 275/40R19 Michelin Pilot Super Sport, HAWK HPS + ATE DOT 4, Rear CS style pedestal spoiler. Eleanor style side scoops, "C" style side decals, Black AM BOSS style roof vinyl, black hood custom vinyl & Black pony.