Do you know anyone with a GTI to test the stock Austin fitment?Konig Rennform's do need a 5 mm spacer for a perfect fit. I started with a 10 mm spacer and found it was unnecessary, and that the caliper actually looks better when it is closer to the wheel.
Stock Golf R Pretoria's 19 x 8 ET 50 fit perfectly with just a 10 mm spacer. I almost sold these thinking they would not fit with this caliper based on the OP, but that was for the 18 x 7.5 version so... If you already have spacers for a flush look, then you are good to go already.
I also saw it mentioned that you can't use the spare doughnut with this caliper, I don't know if the GTI spare is a different size, but on my golf R with a 15 mm spacer it fits just fine. I was using some 5 mm spacers and at 10 mm it just touched. So if you don't want to be swapping tires back to front on the side of the road, just get a 15 mm spacer and stow it with your spare. I found a 64 mm stud fits really well for up to a 10 mm spacer with most wheels and fits perfect with the spare and a 15 mm spacer.
I have already had to use this spare on my front and it works just fine, and no where in the manual or on the tire does it say not to use it on the front. More often than not it will be your front that ends up going flat so...
Getting these calipers prompted me to finally do a stud conversion, and now that I have I am kicking myself for not doing it sooner. It makes everything so much easier to deal with, especially if you are contemplating this caliper upgrade or even just playing around with wheels and spacers, just do it. It doesn't even have to cost much, I found these studs and nuts on amazon, all in was > $50 and I have had the wheels on and of several times since installing them and even did a track day and they are holding up just fine. One other thing the spare is a flat steel wheel so doesn't really have a seat, so whether you have ball or conical nuts they both are fine to use with the spare.