Er... I'd take a punt that you were running very lean under boost; that's the one that ensures you get a linear increase in fuel pressure with boost. Without it being attached to anything the fuel pressure just stays at 3bar; so unless you had it mapped to give more and more duty cycle to compensate, the ECU wouldn't be throwing enough fuel into the cylinders.
Obviously I'm happy to be corrected if I've got that wrong, but I'm reasonably sure that's how it works.