It would be interesting to see some flow data, plugged and unplugged. I do know that hole was put there for a reason, for venting the back side of the diaphram. The vent allows air to escape and be drawn in as the pump diaphram cycles in and out. A pump without a vent (closed cavity) will have a dampening effect on the back side of the diaphram. This is beause as the diaphram moves into the cavity, back side pressure increases. When it pulses the oposite direction there is vacuum in the back side cavity. This slight pressure change can dampen/slow the diaphram pulse. Is it enough to make a difference? Maybe, if your close to using all the fuel that the pump can deliver, and make a change that could reduce the output even slightly. On a "long" WOT pull, the float levels could start to drop causing a lean condition. Backing out of the throttle slightly would reduce the engine's demand for fuel and allow the pump to "catch up". Larger main jets, puts more demand on the fuel pump, because it has to feed even more fuel flow to keep those bowls at the proper level.
Fuel pumps that are designed with no vent hole will still have this dampening effect, however the engineer will factor this in, when he/she designs the pump. This can be done by making the pump larger, larger back side cavity, etc.
A properly operating pump will put out it's rated flow, with or without a vent hole, if that was the way it was engineered. Make a change to it ,, hard to say what it will put out.
My sled is piped, I figure why take a chance, when elevating the pump off the tunnel fixes the problem.
Whew! sorry guys, I'll get off the soap box now!
