[churchx]

Blue Bird: Drop-in filter shouldn't affect AFR.
What were mentioned in your linked post as doing that, were intakes, that has such shape/size/MAF sensor placement, that allows passing more air while making MAF sensor by it's stock calibration misdetect as lesser air volume, thus when ECU tune adds in fuel amount matching those mis-readings, result is leaner air-to-fuel ratio. More power, but also less knock safe. Better doing proper ecu tuning, not hacking-around. It happens though, that after proper tuning for new bolt-ons, taking into account changes to MAF sensor readings, large share of these "hackish gains" by artificial leaning ARR are diminishing, making those parts to add way less power, by price/performancy. There are also such intakes kits, that had goal when designed, to not change stock MAF sensor readings (IIRC Grimspeed & TRD ones should be such), but of course they have less "gains" with stock tune, thus may look less competitive.
Changing filter to less pressure-drop/less resistive - leaves everything else stock. Airbox/inlet tube/MAF sensor placement. Even if air with everything rest being same can flow through easier, MAF sensor will detect that larger volume of air, and ECU tune will know to add more fuel in to reach targeted AFR.