You want to be between 10 and 15% boil-off per hour.This is interesting number because commercial brewers are targeting 3% evaporation rate.
I read the Siebel document.
This is because they can.
The amount of water that is evaporated during the boil is directly proportional to the energy that is used for boiling. And ever since energy became a major cost factor in brewing brewing equipment manufacturers came up with ever more efficient kettle designs that allow the same level of DMS removal with ever lower boil-off rates.
Back in the 1970s they has so called "high power" kettles that could run evaporation rates in the upper teens.
There are only 2 out of the 5 "ations" of the boil for which evaporation is needed: volitiziation (sp?) and evaporation. The latter determines how much additional sparge water can be used and with that it is linked to the efficiency. One can actually calculate at what point the savings in grain equal the energy needed to boil off the excess water needed to get the malt savings from increased efficiency.
The more important one is the volitization part. The less you boil off the less DMS will be stripped from the wort. And since most of us have simple cylindrical kettles we can't use the evaporation numbers that a brewery with an internal cooker and possibly post whirlpool DMS stripping can run. Dr. Fix referred to a minimum of 8%/hr boil-off for sufficient DMS removal. I'll have to dig up other references that list such a number but I found that keeping the boil-off between 10 and 15% is a good compromise between making sure that DMS is sufficiently removed and that the wort is not boiled too hard or too much propane is used. I tried to push it down around 7% once and found that the resulting beer was a bit more sulfury than usual.
Tygo, the DMS stripping ability of the boil is actually tied to the boil-off rate and to the cooker design. As the wort size increased the boil-off will have to remain the same unless you also change the way the cooker is designed. Modern boil systems, like the Stromboli for example, spew out the wort as a large umbrella where there is much more surface area at which the DMS can escape.
Here are interesting PDFs on this subject: