Increasing gain with increasing power sometimes happen in AB class power amplifiers, because in AB class the conduction angle depends by input power. The phenomenon it is named, Gain Expansion, and this is dependable by multiple factors as, Icq quiescent current, AM-AM and AM-PM interaction. Usually gain expansion happen few dB's back-off from P1dB of the PA.
The theoretical maximum efficiency of Class-B is 78.5%, when Class-AB is somewhere between 50% and 78.5%. So, they can be equal at some point.
You replaced the output balun with a transformer, but what about input? Linear gain of the amplifier depends also by the input match, balance, phasing, etc.
If you cannot reach the 15dB gain (with low input power) with none of the configurations, maybe is something wrong with the spice model.