I had an impreza turbo before my S3. Both had been remapped, so like for like comparison.
Always averaged 23mpg in the scoob, but seem to get 27mpg in the S3.
(thats mostly 10 miles commuting to work each day, with maybe 50 miles or so at the weekends)
On a long run you'll see 32 mpg easy with the S3. Travelling at 100mph all the way back from the Ring to Calais last year saw 35mpg.
The Impreza would have been lucky to break 28mpg.
Also the servicing costs are a fiar bit cheaper than the 6 months or 7.5k miles that you'll have in classic turbo.
I'm on AVS (roughly £350 a service) but that comes round every 2 years or 18K. I do change the oil every 6k though to be extra careful.
The Impreza used to cost me £120 for just an oil change + the medium service was £350, so roughly around £500-£600 per year.
The running costs start to make a reasonable saving. Also factor in the less desirability of the Impreza now, and re-sale time should see you a few quid better off to when you sell the S3 later on.
The only thing I miss about the Scoob is the flat four rumble, that cannot be beaten. However, I don't miss the image that the early model Impreza's are now attracting