You are on the right line of thought. The resin will have a load capacity, once reached it won't pick up any further lithium. So higher ppm will mean more frequent wash cycles which could actually reduce process efficiency, although production efficiency might stay the same, or potentially improve.
Think of the treatment train as a connected collection of levers and pulleys. If you adjust one it affects others. Optimising the process is about tweaking all the pulleys to find the most efficient mix. There will be a design envelope within which different metrics can vary to some degree at an acceptable efficiency. The plant is designed to produce 10-13ktpa. To fully exploit higher ppm they'd likely need more modules (i.e. a bigger plant) so that they can produce more than 13ktpa.
