I did my 850r recently too (front pads and new rotors from IPD, rear pads only and I have an sqeal now and then but I've noticed its from the rear, is your from rear also? IPD sells a shim kit for the rear and they have anti squeel grease also or you can get this expensive anti squeel grease from dealer. I did not put anti squeel on both sides of my metal shims(rear) though (mental mistake) and I'm almost positive thats where I messed up. If you don't grease them then corrosion builds up when your car sits any period of time. So I would target the rear pads as the culprit especially since you did not get new rear rotors and I'm assuming you did not have them turned. I did the same thing assuming they were okay but if the rotors don't have a good surface it can cause a vibration that=squeel.