I haven't looked at the parts catalogue yet as i dont have an F30 but i have a feeling the F8x has a specific knuckle which is specific to the suspension geometry, like the F2x M135i/M235i has its own knuckles with 0.5* extra negative camber over the standard F2x knuckles.
The F8x knuckle might also have its caliper mounting holes cast with an extra ~5mm offset from the hub centre to allow the bigger rotors to fit. This allows BMW to save costs by using the same caliper as the S2NH kit. Which also means non-M cars get M grade calipers
