I have seen these wings in the flesh and they look awsome. There are 2 versions. One with the 20mm increase in width and ones without. OSIR is based here in Hong Kong and my local garage has fitted about 4 sets to A3s. He told me that the quality is superb and most of them have been fitted for over a year without any problems. However, all were the regular ones without the width increase.
I have actually ordered a set of the 20mm wide ones for myself about a month ago and have cancelled the order because of the same concerns with the narrower rear. I have spoken to my local garage to see if there are any solutions to this but a custom rear job is the only option.
I am using 10mm spacers in the front and the wheel just sticks outside the wing about 2-3mm. Once you add the 20mm width the wheel goes in about 17mm and it might not look good.
Another option is to just go for the style without the added width. Waste of time and $ IMO.
Also, you have to think about the shaved side indicators. As there will be NO side indicators. Of course unless you are going for the new LED mirrors or dont care about this.
Hope this helps.