There is a fundamental limit to the amount of algae that can be grown on a screen of given size with given flow and given light from a given source. Word of the day: given
As far as limits go, many of them are generally assumed based upon anecdotal data, but not many have been truly (scientifically) tested. Such as the upper limit of copper presence before algae is killed or growth/filtraion is hindered, actual uptake rates of certain nutrients, I could go on but you get the general idea.
I do know that someone did carry out an experiment of sorts to see what effect iron dosing had on growth. It went so far as to dump an entire bottle (I think 500mL) of Kent's Iron/Manganese into a 90 gallon system in one shot. This resulted in a lot of coral mortality, but it gave insight into the scrubber's ability to handle such a large dose. Not scientific either.
Some of these things are tests I might try to perform once I get finished with my LED experiements.