No Craig, you've a Haldex 'Quattro' system, so there'll be no problems with the 4wd.
Had you a Torsen 'Quattro' system then yes, you'd certainly develop problems.
A for the tyres, a 7.5 to 9J rim is recommended for a 225/40/18 tyre.
However, consider the Audi RS3, this car has 235/35 fronts and 225/35 rears on 19-inch/8J rims and that's the Audi STANDARD.
However, on my RS3 I'm using 255/30 fronts and 225/35 rears on 19-inch/8.5J front and 19-inch 8J rear rims, and that's the Audi OPTION for the RS3.
NOTE: The RS3 has bespoke carbon fibre front wings to accommodate the 'larger' front wheel and tyre; your S3 8L will not have that same arrangement so take care.
Hope this helps...:thumbsup: