I have done a very quick bit of research on 351 brake discs.
Essentially it would appear that most of the 06 onwards X350 braking gear found its way onto the X351. The car was indeed based on the outgoing X350 and tried and tested parts found their way to the X351. Not an uncommon practice w ith motor manufacture.
A very quick look at the attached link found typical X350 355mm discs on the front and an even more typical new unworn disc cross section of 32mm. My own experience with such discs found the minimum disc cross section after use with mileage accumulated to be 28mm afterwhich the discs should be regarded as worn out. Regardless of disc diameter which will inevitably differ from model to model, the starting cross sectional area is usually 32mm.