I have the 18Z at the front and R32 brakes at the rear. The MC is the same as the R32. The pedal feel is great and I can put all my weight in the pedal, it doesn't go to the floor, the car just brakes really fast if I do that on track. Not that safe to do that heavy braking on the road if you have someone from behind. lol
Doing the math I don't think it would be advisable to fit the 4-pot rear brakes without upgrading the MC but you can try to find the feedback from others with the rears as well. With just the 18Z fronts you can find a lot of good feedback. Although I have found a feedback from someone who seemed to be knowledgeable stating that fitting the full setup is needed to balance the rear while under heavy braking.