It's certainly one piece (of MDF), so I can't shim it, unless I did so under the sacrificial sheet, but I have plenty of room to screw it down outside the machining area so I am going to do that and machine it flat.

The table is bowed (high in the middle) and stiff enough that deflection is more likely to be caused by the unsupported rails than by any flexure in the table itself.  I will put the Z axis down into the center of the table when I get a chance and see, but I'm certain it will just deflect the rods.  In fact, I am going to add some supports under the bearing holders just to minimize deflection of the rods because that *can't be good*!