I hope I can help.
A couple of weeks ago I fitted 15mm spacers on my V with 17" Enkei WRC Tarmac Evo. The car is lowered on Eibach springs and I had no rubbing and it sits perfectly. I wouldn't advice going anymore than 15mm as I can see that it's pretty close to the arches. I guess 20mm it's too much, at least at the height that my car is atm.
I bought a different brand of spacers so on that I can't give any advice.
I attach some pictures so you can see how it looks.