It fits!
Direct swap, no cutting or modification anywhere - Z4M stock rotors (345mmx28) and 335i front calipers (2012-2013 335i or 2011-2016 Z4)
Weight difference 3.0si Caliper/Rotor (325mmx25) to this new combo = 1lb heavier per corner. Being that I shaved 2 lbs per corner by going to ARC-8 wheels, still not bad. Although the 335i caliper is made for 348mm rotor, mostly all of the brake pad surface covers the Z4M rotor.
I have yet to calculate piston/pad area for some rough check of bias change. Time to prep and paint the calipers. Just barely fits inside the ARC-8 wheel, but clears as well.