Checked the parts catalogue. The suspension upright (or wheel bearing housing as it is called by VW) is different between the brake types. 17" brakes use 4E0 407 253 R, 18" uses 3D0 407 253 AC. There are even different versions of the stabilizer bars for different engines. Otherwise the front suspension components are the same.
The 17" brakes are really not that impotent, either!

In fact they are used by Audi in various models, from small-engined 2004-2011 Audi A6's to 2003-2010 Audi A8 W12 (!). And as you know, the US Phaetons never got the eight-piston brakes, not even the W12.
The rear brakes are the same in all Phaeton models and are also used in Audi A8 of the same vintage, even including the S8.
As the brakes are not unique for Phaeton, it is pretty easy to source aftermarket brake parts. I just bought new brake discs for all four corners. They are made by Zimmermann, a pretty reputable aftermarket supplier. Cost was about half of the OEM.
Jouko