I don't think this will work. Yes you will get less fuel in in theory, because the O2 sensor and mass airflow will tell the computer to lean it out. The problem is the engine will labor to suck in air. This will require extra power. If your car doesn't just stall out, your mileage will probably go down.
Hmmm I usually call this a dirty airfilter and it usually costs me a couple of mpg...
The "right" way to do it would be to downsize and lengthen the intake tubing. Then you'd push torque at low RPM wayyyyy up and it should pull you into minimum BSFC territory if you plan it for your rev range and driving conditions.
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice