I think you know the answer. Go to that tuner and get the original software installed. I doubt the dealer will complain about any warranty issue if you are happy with the sound again. Obviously it can't be Audi policy but they would be idiots to complain or make a case of it. Look at it this way. If you fight them in court to fix your car you will lose (as you pointed out the judge will not be happy).  If you have a warranty issue as a result that would be a totally different court fight they would rather avoid. Likely hood of it happening is close to nill. So get your sound back. Screw the expensive exhaust option or warranty threats.  This is the same attitude they took with all their wealthy R8 customers.