The higher the octane rating, the lower the flash point, therefore higher boost could be used without causing detonation.
I do not think Navy/Marine Corps units had access to 150 octane fuel in the combat zone of the PTO. Widewing knows more about this than probably anyone on the boards, and could set me straight if I'm wrong. IIRC, he stated in another thread that 150 octane (in combat zones) was limited to 8th Air Force for the most part. So, if they were only using lower grade fuel, they wouldn't be able to pull as much manifold pressure as a test pilot back in the States, i.e. lower climb rate from less horsepower.