I believe it has to do with the traction and stability algorithms used by the electronics. Porsches comes from the factory with rearward torque bias as well as different sized tires and wheels, so the algorithms used account for that. The MS6 doesn't, and arguably the torque biase on the MS6 is front biased, so the algorithms would be set up for this. Putting on tires with greater mass and different shaped contact patch on the rear could put the algorithms into performance zones that they're not programmed for.
However, I haven't heard of anybody actually trying it and confirming that it won't work. If you can find the right combination that will fit into the wheelwells and make it work I'll be the first to admit I was wrong. However, if you've seen any of the other threads on what offset and tire sizes can fit, the MS6 is limited by the rear fenders. As the car sits, it can take a bigger tire width/offset on the front than on the rear, so it's an uphill battle right from the get go. I don't think it will possible to do anything like a staggered set up without body work on the rear fenders, unless the stagger was achieved by making the fronts smaller, which may achieve the "look" but would certainly compromise drivability (major understeer) and handling.
As for using grippier tires of the same size on the back, I'm not sure what this would do. All articles and recommendations I've come across say to use the same all the way around. Again, try it and see. However, as above, using grippier tires on the back will aggravate any understeer already inherent in the car.