Breather mod is bad for gas mileage, even though it may give a slight increase in performance by leaning out the mixture. The PCV takes air from the incoming metered air tube, through the crankcase, then past an oil separator and PCV valve, then into the intake. Eliminating that tube from the intake and replacing it with a breather filter causes unmetered air to be sucked through the breather, through the crankcase, and into the intake. The ECU can "see" this unmetered air by reading the upstream O2, assumes an air leak or modification has happened, then richens the mixture to compensate, hence your lost mileage.
If you want to eliminate the gunk buildup from oil in the intake, do not replace the PCV draw tube with an old school style, pre-PCV hotrod breather. Instead, place a catch can inline with the PCV tube between the crankcase and the intake. Place an oil return drain in the bottom of the catch can, PCV tube on one side near the top, the intake tube on the other side near the top, but slightly offset from the PCV side... Anyone that needs a visual, look up DemonSVT's catch can he made for his Duratec.
