You're correct in terms of input referred P1dB calculation. However, for PA design all we care is how much power you can deliver to the load at a certain linearity level (TOI or ACPR) with given DC input power. Even your input referred TOI may seem better as you decrease the gain but what really matter is what you can deliver to the load.

Your output P1dB is determined by your load-line for given supply voltage. Always back to Ohm's Law  ;D