I was doing something similar a few days ago, fitting new lighter springs into the forks of KG's Monster. We replaced the standard 0.86 to 1.0 kg/mm progressive springs with linear 0.76kg/mm springs. The new springs were shorter than the previous ones and we had to remove the original plastic spacers and replace with longer aluminum ones. As a bonus we also achieved a weight saving of 280g (0.6lb).Long overdue fork and brake service. Someone wants to ride her Sprint
To give some perspective of how stiff the Monster's forks were originally, the springs in them were not only stiffer than the Sprint's OEM springs (0.68 to 0.9 kg/mm) but were also on par with the (0.8 to 1.0 kg/mm) Promoto springs I installed recently (and the ST+Me weigh about 90kg more than the Monster+KG).
Since then we've been for a couple of short rides, and KG is much happier with the new ride characteristics. The ride from the front now matches the rear which has improved since we fitted the Wilbers shock which also has a much softer spring than the original (570 lb/inch vs 750 lb/inch).
My biggest regret about doing the fork springs on both bikes now, is that I didn't do it years ago. It was much cheaper and easier to do than I thought it would be, and has made such a huge difference. For those people wondering about it, talk to your local suspension specialist, and get them to recommend/provide springs based on your requirements. Then watch some you-tube videos, or get a friend with experience to 'oversee' your efforts and have a crack. The ST forks are pretty straight forward, and if you wanted to go for the cheapest method, you could get away with not replacing the seals/bushes since you can access the springs by opening the top cap and dont need to separate the inner/outer tubes ( I went the whole hog since I used forks off the spare bike that had been sitting idle for 4 years). You will need a 1litre bottle of fork-oil which will be slightly more than you need to fill both forks.
A set of fork springs, seal and bush kits and a bottle of oil will cost about the same as 1 new rear tyre.........