Not exactly. In the "Peq Method" implemented in CAESAR II, the Equivalent Pressure (Peq) must be less than P_B16.5 as you say. However, Peq is determined as:

Peq = Pn + F / (pi * G^2 /4) + M / (pi * G^3 / 16)

where Pn is the user defined pressure of the load case in question. You can see this easily by setting up two load cases as follows:

1) W+T1+P1
2) W+T1+P2

where P2 is double P1. Then it should be easy to see in the output that the input pressures contribute to Peq.