Honestly looking at the OEM brake pads, I can't really tell which one has less area. The picture you posted doesn't seems to match what I found on bimmerworld.
https://www.bremboparts.com/europe/e...m5/000128183-1
https://www.bimmerworld.com/Rear-Bra...06-F12-M6.html
Like I said I would worry a lot more of messing with the bias by increasing the stopping power on the rear. I am sure Racingbrakes has a lot more experience and can tune that with pads and rotors for you, or the change is so small that might not make difference. Usually your front brakes does ~70-75% of the stopping. In race cars you can adjust the brake bias between front and rear. Let us know how your conversion goes and your track feedback.