2 shims worth is a bit vague
I'm still working on breaking mine in. They were pretty stiff out of the box, but don't feel all that bad after a few minutes. I imagine they'll get better.
From what I understand the PFP is like changing the ICS in a set of KYB/Showas. (The internal spring). Or increasing the pressure on a WP Bladder fork.
Less preload can plushen up the initial action of the fork, while cranking it down will firm it right up. I don't imagine it will be something that I use too much. More likely I'll set it towards the midde and then work on the stacks.