It is nonsence MINIAC is refering to the old single piston JCW brake kit as this has 294mm disc so can fit 16" wheels, the new JCW brake kit as fitted to factory car comes with 316mm discs and only certain OEM 17" and 18" wheels fitIt is not nonsense. The GP wheel does not fit without the use of at least a 5mm spacer.
This is due to the spoke design
It is reported elsewhere on the forum by a US member who has an R56 JCW and a GP
There are other 18 inch wheels that also don not fit.