On a stock or close to stock truck the only thing that might be worthwhile is a bag of parts.
I have considered it, but since i don't have a rail psi gauge anymore I am not sure I could ever see the difference. But when I had the gauge I would notice the rail psi drop when I hit hills and would take a few seconds to recover, a bag of parts would increase the recovery rate, and could lead to better mpgs, or more power at the throttle change.
Big gamble for $400, plus a gauge to see if it works.