I had my rear spring replaced. I didn't do it myself though; I sent the rear shock to Norwest Suspension, who replaced the stock spring with a heavier one and also rebuilt the shock.
I did do a spring-only replacement years ago when I had a V-Strom. I had that done by a company called Cogent Dynamics. I felt that the spring-only replacement without a rebuild did make an improvement. I do a LOT of two up loaded touring, and having the heavier spring did improve the ride.
Both Norwest Suspension and Cogent Dynamics were excellent companies to deal with. If you contact either one, they could provide guidance on what rate spring you're looking for. I'm not sure if either company sell just the springs to the general public, but you could always ask.