The 18Z are a bit oversized for the OEM GTI master cylinder. Going down that route really needs to be accompanied by the TTRS master cylinder upgreade.
I haven't read any bad feedback online about the 18Z from people who actually fitted them, just from others who look at the numbers that really don't match. The GTI can have 2 different sizes master cylinders, a 22,2 mm or a 23,81 mm.
Mine has a 23,81 mm that can be easily seen by looking at the part. My GT Sport has a 22,2 mm MC.
All mk6 GTI have the 23,81 mm MC.
MC list:
-22,22 mm (some mk5 GTI)
-23,81 mm (some mk5 GTI, mk5 R32, mk6 GTI, Golf R)
-25,40 mm (RS3, TT-RS)
My mechanic who is very picky with the mechanical work of every part was happy with the pedal feel and brake power of the front 18Z with the rear R32 brake setup.
In the end I got what I was looking for, a BBK that didn't required wheel spacers at the front, with OZ Ultraleggeras.
With the TTRS calipers I'd need a small spacer and I would have gone for the AKS/Dave B adaptors too (would increase the price of the setup).
Both setups can have cheap enough discs and pads.