It's all about pushing the fluid in the same ratios as stock to retain the same efficiency in the new calipers as they were intended to work.
If the car that you're taking the calipers from has the same sized (and number of) rear Pistons in the calipers, your new fronts will work as intended.
Seems the benefit of an ATS calipers setup is the calipers mountings allow for the larger disc, and the larger disc provides the extra braking power.
That would be a better swap in the real world than a 4-pot caliper setup would be if you never changed the rears.