The Z's fuel pressure regulator is set for about 50#. It's located at the back of the car, near the fuel tank. Fuel pumps from the tank to the regulator, which you can think of as having a spring-loaded checkvalve. When the fuel pressure on the checkvalve is higher than the spring force, the valve opens and fuel is returned to the tank through this bypass.
The pump is not variable speed. It's designed to flow enough fuel to supply the engine when the injectors are consuming at their highest rate (plus a bit more for "margin"). At Idle or cruise, the engine is merely "sipping" fuel. The pump is pushing much more fuel than required and *most* of that excess is going through the regulator and back in to the tank. But not *all* of it.
When the pump is flowing more fuel than the regulator and engine can "consume", the pump "overwhelms" the regulator (the regulator is wide open) and the registered fuel pressure will be at some level greater than the regulator's setting. As you open the throttle, the engine starts consuming more of the excess fuel being pumped by the pump and the fuel pressure starts to drop. But the regulator is still wide open. Eventually, you've opened the throttle wide enough the engine is consuming all the excess fuel being pumped by the pump. At this point, the regulator starts to close and the reduction in flow through the bypass starts to "make up" for the increased consumption rate of the engine.
In normal operation, you should see fuel pressures at the rail at idle that are well in excess of the regulator's setting (which is in the range of 48-52 pounds). As you load the engine and put increased demand on the pump, the pressure should progressively drop until you get to the point where the regulator's bypass starts to close. At this point the pressure should stabilize at the regulator's pressure setting. (or thereabouts. Remember that the fuel pressure at the rail is not what's being measured so some "loss" can be expected up at the front of the car).
If, however, you pull more fuel than the pump can supply, even after the regulator closes completely, fuel pressure will continue to drop. In this case, you need a higher flow pump to compensate.
Based on your description of the pressure, I would say (assuming your fuel pressure readings are accurate) that you are drawing more fuel at WOT than the pump can easily supply. You need either a higher-flow pump (Racetronix makes a nice one. I have one of these) or a Boost-A-Pump (which supplies higher voltage to the existing pump). I question whether the 38# injectors are large enough, too.
That being said, aftermarket electronic fuel pressure gages are notoriously inaccurate (my Autometer gage reads 9Lbs lower than actual). If you haven't measured pressure at the Schrader valve on the fuel rail with a known-calibrated gage, you should do this first before anything else. My A-Pillar gage registers 46Lbs at WOT, which is horrifyingly low given the motor's making 600+HP, except that I calibrated the gage with a precision gage and I know that the 46 I'm seeing on the dial is actually 55Lbs which is just fine given I'm running 42# injectors.
And *that* all being said, an 11:1 AFR says you're getting enough fuel to the engine. I would tend to doubt the fuel pressure readings and defer to the precision AFR you used during your Dyno run.
