There's very few members running an 8L A3 on 19s, I can't see how it won't work, you just run a lower profile tyre, and it will fit. I don't see how raising it a little will help with the airbag going off situation, it shouldn't be going off at all with the type of shock that comes from going over cats eyes. If anything, the ride on 19s will be a bit firmer than than 18s due to less rubber, and the ride height will go up a smidge. Even raising your suspension a little won't effect your ride, the spring rates are fixed, aswell as your damping.