The engine efficiency is irrelevant to the contrail persistence. If the atmospheric conditions are right for this, the contrail will form and persist and, if the conditions are right for this too, spread, regardless of the engine efficiency. And if they are not right, the contrail will dissipate, regardless of the engine efficiency as well. The engine efficiency only affects the life-time of a non-persistent contrail. The more efficient the engine, the cooler its exhaust. Therefore the vapor concentration in the exhaust is less diluted by the atmospheric air by the time when it has cooled to the temperature, at which vapor begin to condense. This means that at the equivalent atmospheric conditions, a more efficient engine generates a denser contrail, that takes longer time to dissipate. And longer time means longer contrails. If contrail dissipates, say, in just five minutes, its length will reach about fifty miles.