Final answer to speed anomaly? Offset paddlewheel. Verified with GPS.
Just gonna have to learn to live with it. The rig has been tightened
according to the instructions referenced by a few responders and the
change in performance of the boat is to say the least, significant. It
is way more predictable on all points of sail. I can point way higher to
windward and the boat doesn't get squirrely on me. Thanks to all for the
tips.