Power stages in Amplifiers generally do not have such high gain as driver stages often do, and are usually selected in a particular form because they are more efficient, rather than for their gain. Because of that it is sometimes necessary to use a driver stage to obtain the drive level to the power amplifier to get the output power that you want, to achieve the overall gain.
So yes, a driver stage is there to increase the overall gain. Sometimes it also provides a bit more isolation of the source of the drive signal from the load (e.g. antenna) which may vary somewhat in impedance.
Usually a driver stage has minimal impact on parameters such as Psat and OP1dB of the final stage, as they are parameters describing what is happening in the output stage.
Input levels to achieve 1dB compression and saturation will of course be affected, as now there is more gain in the amplifier circuit. I hope that is a better aswer to your question.