The ECU will adjust pretty much instantly once you're in closed loop mode to maintain the air/fuel ratio. Ignition timing won't be effected with a simple filter change.
Strict A-B-A testing is the proper way to test this mod. Tank to tank has way too much irregularity to see a solid +/- 1 to 2 mpg gain or loss.
A good read:
http://ecomodder.com/forum/showthrea...ery-11445.html