I have the armrest and whilst it's useful at times it's terribly designed as it covers the useless cupholder and the handbrake meaning that you're constantly having to lift it up and down. And then there's the cupholder it's self... Why oh why is there only one tiny one? It's a real ball ache on longer drives. I have a 3 door and the space for passengers in the back is woeful, especially when compared to something like your old fiesta! incidentally, I ran an old 02 plate Zetec for many years and I completely agree with you, they're such competent, comfortable cars.
But, for me, it's the driving position and drivers seat on the A3 S-Line that I'm most disappointed with, I just didn't think to check for Lumbar support when I bought the car from West London Audi and they only let me take it out for about 15 minutes. It just isn't comfortable and I've had to buy a Mackenzie lumbar roll which has helped massively.
I've done a fair bit of playing around and also some research. If you're after the best position it's this: Seat lowered as far as it will go. Steering wheel lowered as far as it will go and then the drivers seat pulled slightly further forward than you'd have it usually – This keeps your lower back planted into the rear of the seat leaving little gap between the seat back and your back.
Despite all of my complaining, you're absolutely right, it's a handsome car, especially when cleaned up! (Mines currently filthy). As I've said above, because I only bought it in December I'll be keeping it for a couple of years to justify the expense so I'll have to learn to make my peace with all it's foibles in that time.
I plan to do a few small jobs to improve the aesthetic further, I'm going to get the chrome radiator surround and fog surrounds blackened, I'm also going to lightly blacken the windows and then finally have the wheels powder coated in gloss gunmetal!