I have a 2.93 diff with an S52 swap and I get about 26-28 mpg on average driving around 70+ mph. I've probably logged more miles than any swapped member over the past 3 years. The odometer just went north of 130k so I'm covering about 28k miles per year. I get the best mileage during the summer months and could easily get 30 mpg driving about 60 mph or so.
I was also close to pulling the trigger on a Fiesta ST but I can't stand to let the TI go just yet.