Wouldn't it be dangerously lean with such a low fuel pressure? The ECU measures out fuel by varying the injector-on time, assuming a constant pressure. If the pressure is too low, it doesn't have a way of knowing it and compensating. With a pressure of 75% of the regulated value, the AFR could be as much as 30% higher than the intended target. That would explain the sluggishness for sure, but I'd also expect other signs of trouble, such as pinging or even engine damage.
No, that is a pretty common misconception. When the fuel pressure drops you generally go rich on most
circumstances. This is because as the pressure drops the fuel stops atomizing on delivery and more fuel gets through just at lower pressure.
Think of it like a garden hose... if you put your thumb over the end of it the pressure ramps up and less water comes out, you take your thumb off and it pours out in huge volume. Same thing on a FI system, when the pressure drops volume increases. There are exceptions I've run into over the years (tech by trade), but few.
Generally when you see a pressure issue resulting in lean conditions it is an over-run condition where the pump/pressure is fine, but at high demand condition (accelerating at 5k+ etc) the pump can not keep up and pressure drops, engine leans out. The leaning out is not a result of actual low pressure though, they are both just symptoms of a pump not able to keep up with demand.
This is not the case in a situation like mine where the regulator fails... if you can't maintain pressure at zero load, its going to run pig rich.