Yeah I ended up putting on some 18s with 325/35/18 mickey Thompsons. So far no complaints.
I think in regards to that code its the toral difference between front and Rear can not be greater than 5%. Stock is a 3% difference so you could probably get away with a 345 or 355 tire in the back with a 25 aspect ratio but I think 30 would be too tall unless the tire is really skinny.