I had a similar occurance a couple of weeks ago with my 2011 M6 GT. Long and the short of it, I needed my steering rack replaced. My car was 500 miles out of warranty and it cost me $1600.00. Good luck with this- let me know how you come out (I hope it's better than I did)
2011 GT M6 Premium- Mechanical upgrades include: Boss 302 Intake, JLT CAI, JPC LT Headers, Off Road X Pipe, and mufflers, 410 Gears, Granatelli Hot Street Coil Packs, 93 Octane Ghost Cam Tune by Jon Lund,Eibach ProKit Springs,shocks, struts and sway bars, BMR LCA's, Strut Tower Brace, and adjustable Panhard Bar, Maximum Motorsports Camber/Caster Plates, Barton Shortshifter and bracket, Wilwood Superlight Brakes w/6 and 4 piston calipers, Steeda Spyders- 9-1/2X20 with 275/35 and 11X20 with 315/35 Nitto Invos.