Hi Zulkarnain.Good Morning. Greetings from my side.
Well i think you have confused yourself between gain efficiency and power. As i had mentioned that for higher gain system becomes slightly instable and this is true to some level. For a system your gain should be optimum and the gain decides efficiency no doubt but in practical systems gain needs to be of optimum level else non linearities exist. I want to clear here that high power is one of the techniques to increase the gain efficiency of the system. So you can relate them that way. I hope in answered your question here.
Regards
Atul Sharma