Looks like HRE got a hold of one. http://www.hrewheels.com/news/hre-the-2015-s550-mustang-6g-test-fitting#.U9kIi_ldWSp
20x11 rear
20x10 front
275/35-20 front (285 & 295 may be a possibility)
315/35-20 rear
We've seen some gains with the 3.5L EB by blending in some E85 to get some of the added resistance to knock and detonation. My Mustang EB will see a lot of dyno time, so I'll keep you guys up to date.