Hmmm, I cant see how it depends on your local conditions at all?
Sub 9 knots is sub 9 knots no matter what...
Diminishing returns is interesting yes, and true.
I have found that above 30 meters there isnt really any gain no matter how big the kites are, unless you got a severe different wind gradient, but if we just talk "normal" winds, 30 to 32 seems to be absolute max.
But I also found the opposite to be true, if smaller kites below 8-10 m2, then shorter lines are better, but when you get down to 18-20 m lines, there is diminishing returns when going shorter....
Meaning a lot less range, and not any advantages, IMO.
Talking about hydrofoiling with the typical bigger wings (only), surf or freestyle.
So when you get down to 6-7 m2 foil kites and below, you dont gain in longer lines no, but you dont gain in shorter lines either.
Depending on riding style, the short lines till diminishing return might vary a bit though.
Unless you need to ride overpowered with your smallest kite of course, but then you miss a smaller size which would be better.
Peter