I am not sure on the boost level on the 1st gen trucks. 15 may be close. I doubt it is over 20. Generally if you do not have enough fuel you can not make boost. I have heard of weak lift pumps causing a lack of power. Not sure what the fuel pressure should be for that style lift pump but as long as it has positive pressure to supply the injection pump it should be enough to give the pump all it needs. Dirty of partially plugged injectors could also play in.