Again, 225/40/18s would be perfect...
Although I would normally suggest 8.0" wide wheels, I like the way Ryan is thinking with the 7.5" wide wheels...
In my view, 8.0" wide works well...and I wouldn't run 8.5" wide as they are too wide and are likely to tramline more (and weigh more!)
7.5s...Hmm..
I've often thought the S3 was overtyred as standard, with little adjustibility or panache...
I also have a Mk3 Golf GTI that runs the standard 7.0" wide 16" wheels with 215/40/16s, and handles impecably...
So, by that example, 225/40/18 will fit and work well on the 7.5" wide wheels, once the tyre pressures are played with...
But here's a thought: Insurance problems aside...how about a set of 215 wide tyres?
215/40/18s? Not a great choice though, by all accounts...
215/35/18s would also work...but I'd avoid a 35 profile on a 215 tyre.
215/40/18 Yokos might be an interesting option...light, precise, better adjustability etc...
Failing that, 225/40/18s are perfect.