I used the 245 35 18 A6s with a 1/4" spacer with no problems in the past. I currently have 245 40 18s on the front, and due to the higher profile on the sidewall, I do get some slight rubbing in the wheel well. I will be going back to the 35 profile in the future.

I suspect if you measure the tread face that makes contact with the road, you won't be expanding your contact patch a full 10mm going to the 255s using the same wheels. The reason I say this is I ran 295 35 19's in the rear this year and didn't get 40mm more tread touching the road over the stock 255s. If you don't change the wheel width, the tires will be "pulled-in" slightly to conform to the wheel.

Interestingly, the same is true for mounting narrower tires on wider rims. I did mount some 235 35 19 A6s on the stock 19" rear wheels, and the tread patch is about as wide as my 245s up front. For grins I mounted these on the front and didn't notice any rubbing (1/4" spacer was used). It substantially provides a wider stance up front. I wonder what the rules committee would say about mounting the 19's in the front and 18's in the rear and registering in "Stock". I love pushing the envelope.