F4i calipers are cheap, there's lots of them, they have a nominally greater pad area by about 10%, and are arched for the F4i rotor diameter and sweep, which happens to be the same as the 919.
Caliper pistoning design is quite complex, and calipers for larger rotors, higher top speeds, and greater braking forces being applied, will have some different design factors as compared to something like F4i calipers.
Just because other calipers will mount up and provide decent or even equal sweep with more pad area, does not at all mean they are the most desirable selection.
For the above reasons, I have a set of F4i calipers on hand to retrofit to my 919.
I may be starting with an 07 GSX-R600/750 radial master cylinder also on hand, but ultimately want to have a Brembo, and if I can swing to start with it, I will.
I'm not yet done with the development of my 919. LoL!