A couple of days after I picked up my 335i Msport I did a very good bedding on the brakes. The car only had about 30 miles on it.
I only get a very slight squeal occasionally when coming to a stop, but when it does happen it's very very light in sound, nothing bothersome.
I'm surprised that the BMW techs don't do a simple trick to the brakes to see if the squealing stops.
Seems BMW didn't put in an "anti squeal shim", or if they did it's not an effective one.
If I were you guys I'd ask the tech to apply high heat black grease to the back of the brake pads, the part that contacts the pistons.
The grease acts as anti vibration insulator by keeping the pad backing plate from vibrating and creating the squeal. The grease becomes a vibration damper.
Every time I've done a brake job on my cars or friends and family, I always use grease and it works great.
I've tried the shims, but it's hit and miss with those, and then I have to apply grease anyway..
Some pads comes with a rubbery type material already applied/glued to the back of the pad as an anti vibe insulator, those seem to work pretty well, but you have to get that type of pad. And, over time that backing tends to dry and loose it's anti vibe damping ability.
Even with the M performance brakes, I don't know if they tend to squeal or not, but I thought I read a couple of posts from owners who have had them installed and I think at least one owner said his M perf. brakes also squeal.
The more aggressive pads on my Msport (OEM) don't really feel any different than other non Msport F30's I've driven.
The most obvious thing these more aggressive pads do is put out a LOT of brake dust. They are as bad as my 135i was with it's beautiful Brembo calipers. I wish BMW would have used those on the Msports front and rear.
6 piston fronts and 4 piston rears. They looked fantastic, well, at least once you ignored all the dust all over the place.
Ask the dealer tech to try it and see if they will. If they do, report back if it works. Can't hurt, give a shot.