My experience of this platform (a 2001 S3)
It is by far the most expensive car I have had to run.
I averaged about 22 - 24mpg (mine was remapped) although I did commute 20 miles each way on a twisty B road so mine was regularly driven pretty hard.
Mine had 120k on it when I sold it and showed no signs that it might be dying.
I did tinker with it a bit and it made quite a difference.
The mods I had
bilstein shocks
h&r springs
poly dogbone (gearbox) bush
custom code remap
DS2500 pads
grooved disks
18inch Kei Racing alloys
the suspension mods and lower profile tyres made a big improvement to teh handling (initially verys tiff but soon bedded in)
the dogbone polybush really did feel like it made a difference to the acceleration.
And the remap absolutely has to be done, it just made the difference on this car.
The brakes I would probably not bother with grooved disks again as the grooves made a lovely noise when they were new, but the grooves filled up with hard brake dust and I had to chip it all out with a hammer and punch before i sold the car. However the DS2500 pads were simply stunning, they must have given me at leats 1.5 times the stopping power after they had warmed just a little.
Other mods that I would have done with more money/time
uprated haldex controller (makes the haldex kick in more aggresivley)
uprated front and rear arbs
That said, I absolutely would not change the fact that I owned this car it is by far my favourite car (not that I have much of a list to compare it to) It was very refined (coming from the likes of Peugeot and Ford) and I always found it a nice place to be.
The sound wasn't that bad when giving it the beans, it may have been perhaps to refined at times (easily get up to license losing speeds and not feel like you are doing them)
Would I buy a 2002 model now though, probably not, the amount of niggly little issues I had from it when mine was just 6 years old is enough to put me off buying again. I was forever in teh garage with problems that wouldnt prevent me from driving it but that needed to be fixed.
One example
I boiled the brake fluid one day and took it to my local to get it bled, he actually made it worse, so I then went to my local VAG specialist some 25 miles away who hooked it up to a laptop in order to sort it, they didnt actually sort it yet charged me £100 for the priveledge, I did some hunting on the net and found another VAG specialist in Nottingham who had ran into problems with this system before, he eventually sorted it after 2 trips but it cost £300.
so all in to get my brakes bled cost me about £500 between the 3 garages.