Non-foulers? Are you talking about something like these:
If so, have they been effective in keeping the CEL light from tripping since you removed the cats? I came across the idea somewhere a while back and it's so simple that it's brilliant (moves the sensors a little ways out of the exhaust stream to slow response) but I didn't know if anyone was actually running them instead of an MIL-eliminator/O2-sim. Did you use two stacked on each rear O2?
Everyone says that the rears don't affect the fuel trims, and they don't directly, but as I understand it, the PCM does run tests using the rears to confirm that the fronts are doing what it thinks their doing so it could indirectly affect the fuel trims if the readings from the rears are off far enough to cause the PCM to apply bad corrections to the fronts. That may explain why some people experience no change in MPG after a midpipe swap (I didn't) and some people inexplicably do.
Did the mid-pipe significantly move the location of the front O2 sensors?
How big a drop in MPG?
Did the gas stations in your area recently switch to a higher blend of ethanol? (that knocked a couple MPG off mine last year)
Did the average daily temps drop a bunch around that time - as Maxpowers noted, MPG drop with really cold weather - takes longer to get up to temp and the colder denser air leads to higher pumping losses too. Mine dropped a good bit when we had a couple weeks of weather in the 20's and 30's recently.