There are two different things you can be doing, one is adding preload, which doesn't make the spring stiffer and is what you referred to. But what you describe doing is cutting the springs to make them stiffer and then replacing the length of spring you cut out to keep the preload the same. That does make the spring stiffer. That is something we would do in the old days when we could not afford to buy the correct springs, or in some cases where the correct springs were not available. I really don't recommend doing that, but technically it can work. If you are going to do that I would probably do it 1 inch or less at a time. If you measure the length of the spring and then cut out, say, 5% of the total length of the spring and replace it with a spacer the length of the spring you cut out that should make the spring roughly 5% stiffer. The big drawback to cutting springs to make them stiffer is you loose the proper end to the spring that allows the end to rotate smoothly on the surface it rides on. That can be even more of an issue if the surface it is riding on is a PVC spacer. It may not slide well on that. I always thought the best way to do that would be to leave the ends and cut out a section of the spring in the middle of the spring and replace it with the matching spacer. But I never actually tried that. Doing it that way would preserve the proper ends on the spring. But it means cutting the spring twice instead of once. Before I ever got to try it I started making enough money for a living that I could buy the proper springs so I never needed to try the middle cut idea.
But springs should be available for the V11 at a reasonable cost. I would go that route over cutting your springs to make them stiffer. Also, have you properly measured your sag and determined how much you have? That would be step 1. Just making your springs stiffer by cutting them without first knowing how much sag you have would be a shot in the dark. Springs are a lot cheaper then a full replacement cartridge. But knowing that you need new springs and whether they need to be softer or stiffer then what you have now would be a matter of measuring your current sag first. Instructions on how to measure sag are on here if you need.
Also, it sounds like you have the one version of forks that Andreani actually makes replacement cartridge's for. Black 40mm forks with compression on one leg and rebound on the other, if I understood you correctly.