ALL things being equal (and I mean all), installing a high volume pump will do absolutely nothing for you except increase the amount of oil bypass through the relief spring and consume a tiny bit more power.
Let me try to explain. Starting with the pump. The oil pump is a positive displacement pump. That means, at a given speed, it will deliver a fixed volume of flow, whether there is back pressure or not. Increase the back pressure that the pump is delivering in to, and the volume stays the same (being oil is essentially incompressible at the pressures we are talking about) - of course the power required goes up. This trend can be continued until either you run out of power, the driving mechanism breaks, or the pressure containing components downstream rupture.
Now the engine. As was already pointed out, the passageways and bearing clearances essentially make up the backpressure "system" that the pump is delivering into. Think of it like a fixed orifice. Try to pump more liquid thu it, and it takes more pressure. As long as the passageways and bearing clearance remain the same, the pressure for a given volume flowrate remains the same. Increase the volume rate, say by increasing the speed of our positive displacement oil pump (ie engine speed), and the pressure at the pump outlet increases.
In an ideal world, the engine designer would look at the oil requirement of each bearing, add them all up, and select a pump that delivered that exact amount of flow. But, that would not leave any margin for wear, for the variance in distribution and of course most importantly, the fact that the automotive application has a constantly varying pump (ie engine) speed. Granted, the oil required does go up with engine speed, but not necessarily in the same proportion as the oil pump delivery will be (which is direct with speed - double the speed, double the oil flow). So how to overcome this?
Select a pump that will supply the required amount of oil at min speed (idle), plus some wear margin. Install a bypass system that will recycle excess oil as speed is increased. Control that bypass amount by sensing the system delivery pressure, and maintain a more-or-less constant pressure. Select that design pressure based on worst case condisitons - the rest will be slightly over-oiled. The simple way to do this is with a spring loaded relief valve.
So, what does this mean? If you take a stock engine, with stock bearing clearances, and install a "high volume" pump (ie a pump that delivers more oil per rpm than the stock one) and leave the relief valve setting the same, then because the "fixed orifice" (ie the engine passageways/clearances) are the same and the pressure is the same, the oil flow to the engine WILL BE THE SAME. The amount of bypass volume will increase by the difference in the pump delivery capacities. But, it does the engine doesn't even know the difference.
However, if you decide you want more flow to the engine, you need to raise the relief valve setting, which will drive more oil thru the engine and bypass less. Interestingly, this can be done with the stock pump, up to the point all the margin is used up and the pump is "maxed out". Not a great idea. And, of course, running at higher pressures can exacerbate leaks. Extremely excessive oil flows can even start to wash babbit off bearings or cause oil film instabilities, but this is more likely in large, high speed industrial applications than automotive.
Bottom line - why is a high volume pump required?
