I would say a nice clean air filter will probably give you about 4 - 6 MPG (based purely on my own figures) boost (really depends on how dirty your last one was!)
Thats a lot! i would have only expected 2 or 3 MPG - must have been pretty clogged (i fit a new (cheap pattern one) one every 6 months)
The ECU adapts to the cloggeg filter (to an extent) re-setting the ecu (watch you don't loose your radio code) can help the economy, and often the performance
Are you noticing an improvement in performance too?