From what I've read about the ZL1 Goodyears, the front 285's were designed to reduce understeer in the car, while the rear 305's were designed to gain traction on launch and put all of the ZL1's torque to the ground. The reason for having the 285's on all 4 corners was to further reduce understeer in the SS/1LE. Since the LS3 doesn't put out the torque of the LSA, there's not as much benefit to the rear tire tread.
All-in-all I'm extremely happy with the 285's all around. The rears don't look nearly as bad as some people say they do stretched over the 11" wheels. I think I've had 1 person even notice it so far.