I've got 29000 miles on my 2008 MS3GT and have logged every tankful using an Excel spreadsheet. I drive mostly highway miles commuting three times per month between Tallahassee and Orlando, FL. I generally set the cruise control at 80mph. I buy the vast majority of my gas at Costco and was told that, at least in Tallahassee, the supplier is Amoco (unconfirmed). So far the overall average MPG has been 24.52. I have gotten as much as 29MPG.
A factor that is very significant but often overlooked is the effect of wind -specifically headwind component- on highway mpg on a turbocharged car. I you have the cruise control set at your "normal" speed and a headwind exists, you will not notice the increase in boost that the ECU will command to maintain your RPM. I have noticed differences of up to 5mpg due to a 7 or 8 mph headwind at 80mph cruise speeds (approx 3250 rpm).
In general, I was hoping for slightly better mpg in cruise, but realistically, I think 24.5 is a fair price to pay for a car that has such unusual performance capability.