Custom mod MBP to reduce weight?

Those cases are typically milled to accuracies within a thousandth of a millimeter for an ideal balance of weight versus thickness.

If you try to remove any material to further reduce weight, you are likely to seriously compromise the integrity of the case.

You could always try forging and milling your own case that is trimmed down to the same tolerances (or better), using lighter materials. Maybe titanium?

Magnesium case. Call it the NeXTbook