No, the mileage aspect is in the jetting/tunning. The pump shot is to cover the opening of the throttle blades letting in more air and causing a lean condition which results in waek acceleration or engine sputtering out/dieing.
the pumpshot introduces more fuel to take up room to keep balance of the air fuel mix from going to lean. Once the throttle blades open up, alot more air is introduced into the intake/engine. Without the extra gas, the engine runs lean and dies or becomes week.
To much fuel results in black smoke out the tail pipe at worst, slow acceleration if just a bit much. The amount of fuel introduced by the pumpshot should be just enuff to produce a strong get up and go. The amount of fuel savings and resulting mileage is very minor.