I get 21- 22 mpg in my 2000 RS if I'm just driving around town (very similar motors). My car's got 100k+ miles, and I'm pretty heavy on the gas, but that always seems low to me...
Only on long-haul interstate trips can I up it to around 26 mpg. I think my highest ever was 28.
Octane rating should have no effect on MPG. You'd just be paying more.