I put my shim in my FPR around 2001. It was making 73psi at that time. When I removed the fuel bowl three months ago it was making 69psi. Does the shim work? YES, at least it did in my test truck for the last 5+ years. Does the factory spring last? YES, at least according my personal testing over the last 5+ years. Does the spring fail prematurely due to the BB/shim? Not that I have herd of on line or in the last 5+ years of my testing.
You will have to make a decision to try the free shim or buy the $20 spring. There are a lot of shimmed FPR's out there however I have only herd of a few that run the GDS spring.