Just changed my 314mm set up to 345mm today.
I have noticed that the callipers are the same on the 314mm setup and the VXR setup. The only difference is the calliper bracket and disks, even the pads are the same by the looks of it.
So you can change up to 345mm from 314mm by getting the brackets which are £75 each (GM employee price) plus disks and pads.
Can some tell me if I am wrong?