I have read that at WOT, the car goes into open loop and the fuel pressure is not controlled by the computer; it is only controlled by the setting of the fuel pressure regulator. Now, with an adjustable fuel pressure regulator, the car may not have the proper air/fuel mixture at WOT.

I don't know if the computer adjusts the fuel pressure regulator directly, thus meaning that it can decrease and increase pressure. When setting an adjustable fuel pressure regulator, do you want to set it to run optimally at WOT, then let the computer change that setting for regular driving?

 

The computer does not control the fuel pressure at all. It's all done by vacuum. (If you look there is a vacuum line running from the plenum to the FPR). The stock regulator has a spring inside of it that controls the fuel pressure at WOT. The presence of a high vacuum (low throttle position) during normal driving will lessen the fuel pressure.

The aftermarket AFPR's come with a screw that allow you to effectively adjust the base setting of the spring pressure (by setting the preload on the spring). The vacuum hose is still hooked up, and still changes the fuel pressure accordingly.

You want to set the AFPR so that you get best performance at WOT, and let the vacuum adjust it for normal driving just like the stocker did.

 

As stated above the ECM doesn't touch fuel pressure its purely mechanical. However you are correct, at WOT the ecm returns to Open loop mode (power enrichment mode) where its fueling off of tables and not off of 02 readings, this is all a function of the injectors and pulse widths. The injectors will pulse more/longer with a high TPS reading and RPM reading, if you're in WOT your pulse width isn't calculated by the 02 sensors readings.

As far as fuel pressure its pretty much only a WOT tuning devise.. Being as I've experienced both I'd recommend prom tuning over an AFPR, and thats even considering that I made my own for the price of a bolt.