Efficiency is a crucial military value that determines the real values of a unit in combat. Because of budget cuts or general unpreparedness a unit could perform significantly worse than it's attack and defend values would imply. And likewise, a unit in a well-funded, well-cared-for military might do better than you would expect. Not only combat statistics are affected by effeciency, but also a units stealth rating and spotting strength. For example, a submarine with High effciency will be able to remain hidden longer than the same submarine with low efficency. Anti submarine units, like destroyers, will detect hidden submarines more easily with high effecincy than the same destroyer with low effecincy.
Efficiency is mostly a product of funding, so the real question is how much are you willing to pay.