I just had the same problem when I replaced all 4 brake pads last week. Strong vibration comming from the rear end & affecting the entire car at low speed and light-moderae braking. I had lubricated the front pads with brake grease but not the rear.

The fix: I pulled the rear pads & lubed them. Problem fixed.

I normally replace the brake system with the following recipe: All pads when 50% used, Rotors every 2 pad changes, brake fluid changed when rotors are changed or Q2 years.

The 850 rotors warp easily and pads should be properly Bedded when installed. Proper torque to all lug bolts, always. I keep a torque wrench in the car just for that purpose. Warpage risk is increased as rotors wear and this degrades braking performance therefore you should practice replacing rotors on a schedule before they warp or get too thin.

Remember: high performance vehicles require brakes of higher performance; brakes & tires are more important to the car than all other parts combined.