Those x 4.0 and 5.0 riders, FYI, the future shock as delivered may not have a spring pre-installed. Mine came without from the LBS and I had to ask for any parts/manuals prior to leaving.
Threw up a poll on ******:
I put in the medium spring and it made a difference on todays ride. You'd think adding a spring would make it stiffer, but it actually increased front-end compliance. The FS uses a # of springs internally (booster/tuning spring, progressive main and top-out), from what I've seen in a Specialized Deep Dive video. Usually springs in series reduce spring rate, but without knowing the specifics of the FS... it just guessing. Alternatively, the booster spring is keeping the FS up higher in its travel where the progressive main spring is softer.