which boats are considered "yardsticks"? And what if that boat's design changes (as part of an OD rule, etc.)?

Say the F18 is the "yardstick" and they allow a modified sailplan as part of their OD rule... or a carbon stick. If either of these change the sailing characteristics (speed, angles, etc) wouldn't it move the entire PHRF table?

I think there was a blog about this issue discussing the Farr 40 along with Time on Distance vs. Time on Time systems.

Being relegated to PHRF myself (not a fan, but not sure I can convince anyone of the merits of another system) and rarely racing OD events, which system best combines both monohull and multihull fleets? I'm not even going to touch foilers at this point because I doubt I'd see a Moth, G4 or N17 anywhere I sail...

I think in my PHRF area I rate about the same as a Melges 24 and a Hobie 16 non-spin. Which seems a little odd?