Interesting that you think the GTI is Q-Car understated. I find it quite a bit more extrovert than an A3 SB. Certainly it appeals far more to the younger generation that I've been exposed to.
I personally feel that the A3 stands apart from the Golf in the driving experience and quality stakes. They might be similar in terms of on-paper specification but the A3
feels decidedly more upmarket both in the static and dynamic experience. The ride is excellent and whilst it's not as tippy toe as the GTI it does have more grace and composure over rougher surfaces. It's an altogether more luxurious and comfortable experience without being flabby and "un-hot-hatchlike". It deserves its place because, for the more, let's say, "mature" buyer
it warrants the premium over its VW sibling.