Facet pumps dont have the flow volume to keep up with the demand of the IP. They are generally fine at lower RPM but their pressure drops as engine RPM increases. There was a study done on here some time ago, might have been by Agnem, I dont recall exactly but the pressure of the Duralift pump would hit 0 right around 3,000rpm, meaning the IP had overcome the facet's ability to move fuel and the IP was effectively sucking fuel thru the pump.
How that effects real performance; the pressure of the lift pump influences your IP timing. I believe, as the lift pump PSI increase, it advances the IP timing, and as the lift pump PSI decreases, the IP timing retards. The issue with the Facet losing PSI all the way down to 0 is that the IP will lose some degrees of advance, resulting in a loss of power in the higher RPM range.
The real question is how much and does it really matter? I seem to recall as much as 1 degree of timing loss per PSI loss. With a 6ish PSI output, that means you could retard timing by 6 degrees. I dont think its really well correlated or documented but that number sticks in my memory. Whether or not you really notice this is anyone's guess. Some might, some might not and if youre not really pushing your truck hard, it probably doesnt matter.
However, this is why the mechanical pump is so good. It produces a static pressure, but a dynamic flow, because it is mechanically driven, as your RPM increases (and your fuel demand) the mechanical pump also accelerates, putting out higher volume to match your demand at all RPM ranges.