Agreed - if you have park assist and ACC you have the right components to change a stalk and re-code to enable active lane assist - but checking back through D4 brochures you could only get park assist/active lance assist from the face-lift onward (long after the A7).
It is definitely a marmite option, I love ACC but have active lane assist. The steering didn't feel natural or direct (probably as it deliberately isn't) and it was disconcerting on the motorway in my eyes.
If you are serious about an expensive retrofit I'd suggest you try a car with it installed first and make your own opinion