75-85% operating efficiency (design rate/achieved rate over a ship) is an ambitious efficiency most organisations aim for, but many do not achieve, especially for unloaders.
It sounds to me like the original figure you saw was a theoretical figure based on the operation of the machine itself, together probably with cleanup (less for a CSU than a Grab). However, as time went by, this figure has had to be updated to account for the many other factors that influence efficiency, including such intangiables as operator efficiency (varying from operator to operator). Other factors still, derive from the efficiency of downstream equipment that may be matched more or less well to the unloader itself.
From my experience, but without having any literature or formal (and broad) studies to back this up, I generally take 70% as being a real life figure to work with. With some good initial and ongoing effort, higher figures can be achieved, but you probably should not plan on it.
There is no doubt that the last 10 years has indeed seen good advances in the way machines are controlled and this has benefitted the grab unloaders. This, together with the fact that the grab is typically the lighter machine, seems to have given the grab a new lease on life.
Director and Principal Engineer
Mayer International Design Engineers Pty Ltd
Specialist Engineering of Material Handling Equipment,
Cranes and other Custom Machines