Yeah, cutting the spring increases the rate. How much? I dunno offhand. I am sure there is a formula out there. If I had a set of stock springs I could measure thier spring rate , cut them and remeasure.
I dunno how accurate comparing the rate of my current 110K + miles cut springs would be to the advertised rate of new uncut springs. Springs can "loosen" up on thier spring rates as they get used, so stock 100K mile springs are probably softer than new springs to begin with.
Now for some info, ANY spring, be it a leaf, coil or torsion bar, needs to have a shock that is rated to control that spring's wheel rate. So any spring that is not the same rate as stock will be mismatched with a stock shock. Not just cut ones. Mismatched shock and spring rates is what causes the hopping mentioned. Especially when the spring rate is considerably higher than the shock rate. If the shock rate is higher then you get a rough ride.
Also, something to consider. I can buy new springs (stock) for about $67/pr. Which is what? $140 for all 4? It doesn't cost any more, or any less, to make a spring for our application in just about any rate or length. So all those aftermarket companies selling spring kits for $200+ are bending us over.
Steve