The computer has a fuel map that alters the duty cycle to allow more fuel and it the map is off, there are lean or rich spots. Adding an external controller to adjust the fuel pressure, will solve the issue and keep a consistant pressure.
Be sure to not activate this during closed-loop control when the adaptives are learning. If so, they will see this as some drift in fueling and store a long-term fuel correction. Then when it goes in to WOT open-loop, this correction will be used to trim back fueling. We learned this the hard way in the early days before we had complete control of the ECM code.