Looking at pure crash test data, it's said that Isofix and a correctly installed car seat with seat belt offer the same safety. (carseat.se)
Official crash test data isn't available for the Audi A7 currently, though the performance of the Audi A6 (which was awarded the full five stars in its most recent NHTSA crash test) should mean the Audi A7 will offer more than enough protection should the worst happen. (carbuzz.com)