An adjustable fuel pressure regulator can give you more fuel flow but not by increasing the time the injector is open, just by pushing more fuel through the injector when it is open....
Think of the injector like a garden hose with a spray nozzle on it. There's pressure in the line, and a valve on the end. The bigger the opening in the sprayer the more water will get out, this is the difference between a 450cc/min and a 750cc/min injector. The FPR would chage the pressure in the hose... It's like if you turned the wall tap down or up, the sprayer stays the same but depending on the pressure more or less water gets through.
Yes, there are good aftermarket fuel pressure regulators, and no the AFC cannot adjust or control fuel pressure.