Hi Matt.
Great series, I too struggled to get all of a car in focus when panning, I upped the f stop a bit which helped, but didn't solve it completely, I'm guessing that at the long exposures for panning the cars may be exhibiting pitch, roll and / or yaw as well as linear movement and we are only compensating for the linear movement when we pan, also the focal plane is flat and we are moving through an arc thus perhaps requiring a curved focal plane or greater DOF.
Waiting for those who know to correct my guessing! ;D
Cheers, Graham.