Yeah the problem is getting the right combination to keep the VDC/TCS happy, if you have a base model (US) without TCS or VDC then go crazy the ECU wont care, but if you do have TCS/VDC then you have to keep the differences betweenn the front and rear close to stock and that seems to be aroun .4 in on the 17" wheels and .7 in on the 18" rims so if you go with 255/45 that is taller than the stock tire and not sure how the system will react to that, I have gone the other way (too small) and it didn't like it, it would kick in on high speed accel.
So the right choice would seem to be 275/40 and that is where the concern over safety comes in as tire manufacturers suggest a 9 in rim for 275 some have done it on the 17 inch rims which are 8" and the regular 18" rims which are also 8", you would probably be ok on the track rims which 8.5 but is the risk worth it?
Unless you are having traction issues like from Forced Induction I would go back with stock until you get wider wheels.