I tested the theoretical 15 mm more preload result today, and it was pretty good. Green is my theoretical prediction, and red is the measured.
Compared to the ST 0.83 kg/mm spring and 125 mm oil level, the end result is fairly similar. It would be interesting to see how this rode, and what sort of rider it would suit. Overall, the difference between the heavier ST spring and 125 mm oil level and the original spring with more preload is fairly minor. Minor enough to make me re-evaluate my believe that you need to replace the springs. I have always been wary of just adding preload, but maybe that view was wrong. The linear spring certainly gives a more linear result from no load to bottomed, but the linearity is much more dependant on oil level than I would ever have expected prior to having this play. Yellow is the ST spring, red the Monster spring.
Next I took the oil level thing to another extreme, just to see what would happen. With the spring preloaded an extra 15 mm as above, I dropped the oil level to 215 mm. As you can see, it's moving closer to a linear rate overall, but still the rate at full compression is around double that at the beginning.
So to give a visual summary of where I started and where I sort of ended up in an untested guess, red is the start, green the end. I still don't get the desire for such an aggressive progression on effective (assembled) spring rate. Maybe if the initial travel wasn't so soft (giving so much sag) I'd see more sense in it.