I've seen that 60-80% efficiency spec listed too, as well as the tube rectifier spec being quoted as around 50%. But remember that efficiency doesn't have much to do with forward voltage drop. Efficiency would have more to do with resistance at current levels above what is required to bias the junction on. Once there is enough potential to forward bias the junction, current flows relatively freely.The biggest drawback of selenium rectifiers is the relatively low breakdown voltage when reverse biased, not the forward bias drop or efficiency. They also produce a toxic gas when they blow.