Technically, filling up when it's cool out doesn't help much. Your gas comes from an underground tank.
Remember that gasoline is stored underground. Many feet below our wonderfully crusty earthy surface it's cold down there. Ever dug a hole in nice moist underground? It's cold about 1ft down, imagine how cold it is 15-20ft down where they hold the tanks. The only temperature difference would be your car, so you really aren't gaining much of anything. If that were the case the oil companies would be upping their prices by $.20 every night and putting it back down $.20 every afternoon saying it's an "Afternoon special!".
The tires being inflated at max will help you the most.
His underbelly pan (i was reading up on it) doesn't appear to be /air tight/, simply screwed into good spots. If you're that worried, drill a few tiny holes 1 or 2 so it doesn't stay (moisture/water). If you're worried about dirt and mud, etc, take it off once in a while. It appears his belly pan is held in by about 5-6 different bolts/screws, so taking it off to work on it underneath would not be too much of a hassle.
Be careful about your intake. Short ram if it's TOO short can sometimes just draw hot air straight into your intake, you make less power, it's less efficient (i say sometimes, not all cars are the same). And some setups and ECU's will compensate and sometimes cause the vehicle to run more richly (this varies from ECU to ECU).
Remember it isn't necessarily all about how much power you can make, but rather how efficient you can make what you have
