Shimming the Tosa plate should resolve your problem. while you're at it, you might consider actually mounting the Tosa on risers, to create space underneath it, to facilitate cleaning the screw/dowel holes - Kill two birds with one stone.

I would suggest you contact Tormach, and report what you've found. What you describe seems far outside the normal range of tolerance one would expect from a Tormach. Perhaps they will offer some better resolution.

Regards,
Ray L.