Something I learned from Bilstein.
I was so very puzzled by their 600lb spring behavior that I concluded they had valved the 6112 way too softly.
Well, not quite. Their 600lb spring is actually softer and the 6112 is not valved softly. This explains the phenomenal ride quality on 600. And it also means that the 6112 is in fact better suited to the 650lb springs for offroading, it is not just the springs doing the work.
This also explains the huge jump in performance from 600 to 650; it is more than the nominal numbers indicate.
So this is like tires, you can have a 285 tire that is 8.2" wide or 8.9" wide