Unlike most sunroofs the A3 sunroof glass slides back outside the car so no space is taken inside, so there is no lowering effect of headroom.
That's good. Back in 1998 I had a Golf VR6. It came as standard with the normal slideback sunroof, but that made the headroom too tight for me, so I ordered one without a sunroof as my existing one. When it arrived it had the sunroof and as it was one of the very last batch to be made the dealer could not order a replacement, so I regected it.
The only good thing that came from it was I had a look at other cars and decided to buy my first A3. 16 years and eight A3s later and I still love the A3. But my current one will be my last as I don't like the interior of the new A3.