When I remember, I usually either turn on sport mode or disable the traction control all together. Yesterday, I first turned on sport mode as I was coming to a stop with my foot on the brake (double tap button, foot on brake). After I stopped completely, I disabled traction control completely (6 seconds, brake on). After that, the car was extremely loose in the rear. More than it's ever been. This was driving NORMALLY, and I drive like a grandma. Kind of scary even. Slightest amount of throttle around a turn would break it loose. Give it a try, maybe it was just me, weather, etc., but all driving conditions were the same as any other day.

 

I have an '11 GT manual Brembo (same as yours), and it is quite stable at the back in all stability control modes (on, off, tc off, sport). I have done the "sport mode" to "off" switch with no noticeable traction issues. Getting the back to step out is easy, but predictable.

I would check the tire pressures. Maybe you have a slow leak. A seized or stuck rear strut might also cause a lack of rear grip.

 

That's kind of what it felt like, having a flat tire or driving on ice. The only factor that I believe may have caused it was the wind the night before that probably left sand on the roads ( I live in a desert environment). I tried it again tonight and it didn't feel as loose.