It varies by individual depending on the ratio of slow-twitch to high-twitch muscle and other physiological factors, but I wouldn't worry too much about the precise efficiency. Optimum efficiency tends to be at a considerably lower cadence than most of us choose to ride at since there are other factors such as muscle fatigue and strain that come into play. These tend to be worse when a low cadence is used for extended periods at a given power output. The higher gears on your current cassette probably aren't hurting your efficiency by much at all, but they do put a greater strain on your muscles and knee joints. Probably not a problem in the short run, but your current cassette will wear out too and you might want to replace it with one with a bigger cog when it does.