The efficiency of each stage is independent, and comes down to what signals you are passing through the system and how you plan to organize your amplification chain, based on the type of signal you are amplifying (PAR, bandwidth, broadband noise level requirements, etc). You could have a very linear driver (which has a poor efficiency) and a highly compressed final amp stage (high efficiency), or vice-versa... it all depends on what your system is trying to accomplish with a given type of signal.
PADs are simply additional amplifier stages.... and amplifier is an amplifier is an amplifier. You can optimize them to run in different classes of operation, or operate with low noise figure, maximum gain, maximum output power, etc. There are many ways to tweak them, but it all comes down to what your signal power from the exciter (modem) is, and what your required output power and signal performance specs look like at the antenna port. You may need 4 gain stages (including the finals) to get up to the power level you need... therefore you'd have three "driver" stages and one "final" amplifier stage, but again... they're all just amplifiers, optimized to perform their specific job, and in concert, get the signal from point A to point B with the proper amount of gain and acceptable distortion/noise.