Does anyone know how the series 2 Mi16 wheels might fit on an Si? For the same reason as noted above regarding tyres I’m keen to get them to replace the current 13”, plus they’re a lot cheaper than trying to find a set of 15”speedlines (or C5 steelies)I think it depends on state but both WA and SA don't like them. Don't know about others.
Grinding callipers is not illegal as far as I know. When I upgraded mine (on a series 1 GTI) I had to use the Si hubs and I seem to remember there was only very little contact so I don't think there would be a lot to grind off. You could of course take the callipers to a shop and have them machined nicely. It's not a hard job and shouldn't be expensive. I didn't have my milling machine back then, but now that is what I would do.
On the other hand, there are much more tyre options with 15" rims so that might be a consideration too. See Peter's (4cvg) threads.