I will be honest, I have not checked mpg that closely.
But when I did examine it on a couple of occasions it was suprisingly good and MUCH better than my previous S5. The range when I fill up says 350 miles and the S5 was only 290 miles.
I am using Shell V Power for the RS3 though and the S5 was fine with regular 95 ron.
So I am paying a premium for Super Unleaded fuel now.
If you drive it with care and only boot on a few choice occasions and rest of the time treat it like "a normal car lol" then you can get 35 mpg with ease. it is 7 speed remember so you can tick over at very low rpm @ 50mph to 70mph quite easily.
However, go for some choice demo laps and show people what the car is capable of, or drive it round in dynamic and the mpg can plummet big time
Overall I am very impressed at the mpg and it is much better than my last car, albeit that was an older 3.0 litre lump.