My next tank was 55.1mpg.
I don't think holes in the rear bumper fascia will help much, if at all.
Do you have a ScanGuage or UltraGauge? The radiator fan comes on at ~204F -- I'm sure it would be running with or without a grill block.
I need to have the same final drive as your car -- 2500RPM @ 60mph would be great.