I average around 260-270, and that is a mixture of town driving with some longer runs on motorways and A roads. I've had less than 200 but I've also seen 300. All depends on the type of driving. I did a track day not long after I bought it and averaged 9mpg! A bloke who I got chatting to was averaging 11mpg in his M3, but I guess I was having to work the ST harder to keep ahead of him.
Bloody love my car, it's a PFL 5 door ST3 in black running a Superchips Bluefin stage 2 map with a 3" downpipe, 200 cell cat and a custom made cat back. Oh and the RS clutch (cos the standard ones are made of chocolate!). Had it 5 years now and I still love it. The five pot combined with the exhaust makes a great sound.
Incidentally, even though it's mapped, my mpg has not been affected.