You won't gain MPG from a aftermarket intake. If the filter is open to engine room heat you may see a very small increase.. but not much.
There are more effective ways to gain efficiency.
reduce drag. (speed, underdrive pullies, tire pressure, ect..)
Put less gas in. (X-cal tuner, HHO, ect..)
Reduce rpm (taller tires, hwy gears, ect..)
Also, if your exhaust is still in good shape.. there isn't really anything to be gained there either.
See my web page for all my mods. (I avg 21-22 summers and 19-ish winter in 50/50 driving)
He who is enslaved to the compass is free to sail the seas