With that offset you probably wouldn't get any rubbing on the inside arch, it would all be on the outside if there was any.
Standard A4 s-line wheels are 18", ET43 235/40/18. So you're effectively moving the outter edge of the wheel out by 14mm. You can measure the difference and see what the poke would be. Another way to reduce the rubbing out the arches is to use slightly thinner tyres so the tyre wall is angled and thus reduces the risk of rubbing.
If you have 19" ET29 you'd probably run 245/35 not 245/40, so the rolling diameter will be (near as damn it) the same. Therefor if there is no stretch on the tyres the chances of rubbing with 18" would be the same as 19".
That's my understanding of it at least, other people run far greater offsets than me so no doubt they can chime in and offer their 2p. Do you regularly have a full load in the car? If 90% of the time it's just yourself you can probably live with occasional rubbing.
If you're into your modding, changing the s-line springs is something which is WELL worth doing as the ride can be greatly improved plus it decreases the arch gap. If this isn't for you then it's irrelevant, however if it's something you might do down the road you might want to think about using a 'safer' offset just in case.
Remember: You can always decrease your offset (lower ET) with spacers, but you can't increase it (higher ET) without changing you wheels...