That forum is awful lol. I tried my 18s I had on my Gti and got some Yokohama geolander tires in a 265/65/18 and it seemed like I had room still. I don’t even think I want to go that big with the tires anyway.
It really is a great road trip vehicle. I’m interested in up sizing my stock tires when they are due to be replaced, I think I can put a 255/55/20 on there safely and it will help fill the wheel wells.