Sorry, I'm using "preload" incorrectly. Should say "sag" instead since that's what we want. Some amount of compression with no load on bike. Apparently RaceTech spring is still too stiff to allow any amount of sag.
Since you have decent ride-quality when not hitting big bumps, I say next spring should be 300-lb/in linear-rate. Will be same rate as stock initial-rate and not double itself to beat you up on bumps. Does top of shock connect to frame or to swingarm? Looks like they copied Honda's ProLink or Unit ProLink linkage.
Here's Ducati Multistrada V4S