If you actually measure them there is next to no progressiveness in a progressive spring when new, and what little there is sags up solid PDQ.
So you end up with a shorter single rate spring with fewer coils, which is much stiffer.
What you want in the forks is a spring that gives about 10% less rider sag than the rear shock spring, when the fork spring is preloaded somewhere between 14 and 18 mm.
Just about the only way to get that is to go to a decent suspension specialist , who has a few springs in stock and can remove the existing and fit the correct ones on the spot.
He would probably be able to fit properly shimmed 'Gold Valves" or similar at the same time, and if he can this will make a huge improvement.
Experience is everything here - you could fart around forever and never achieve what an experienced pro will do in half an hour, and it is much cheaper than any alternative.
If your forks have seen a decent amount of miles and are getting a bit worn a billet top triple and tubular lower brace will improve the action too, but only if both are carefully fitted so that there is the minimum of stiction - just bolting them on as supplied with the forks still on the bike will probably make things worse.
The 440,000 km old 36mm forks on my G/S with these mods, are stiffer and work better than the 75000 km old 42 mm forks on my Funduro , yet people have fitted Funduro forks to a G/S and claim a huge improvement in action and rigidity - go figere!
