I have worked out that on a 9" rim with ET42 (the highest you would be able to get away with without having the wheel rub on the strut) that you could fit a 255/35/18 tyre and it would overall package would sit about 18mm further out than your TD's with 225/40/18's
Obviously if you went for the 245/40/18 then it would give you a little more room (-5mm) but a 245 section tyre on an 8" rim is not a great idea, the wheel will not be able to support the side wall and you will get a lot of roll in the tyre, also 245/40's are pretty tall in comparison to a 225/40
Im going to try and get the 255/35's on the car but its going to mean new wheels, which are a mission to source, 18x9's with high offset are pretty much non existent with a 5x112 fitment.. Also im going to have to get wider arches on the car, should really help with traction though..
I had 235/40/18 R888's on the S3 with the Speedlines, 18x8 ET42 and there was no issue with them, but even a 235/40 tyre on an 8" rim is a little wide..
Jonny