The way the pump pickup is in the tank, the fuel pump doesn't have to work as hard to get fuel to the engine as it would if the level were lower. If you have a high mileage engine, and you have a severe drop in performance with a lower fuel level, you're probably due for a new fuel pump, You can go back stock or upgrade, really doesn't matter, the stock fuel pump will support up to around 300HP, though obviously being strained pretty hard to do so.