The flatter your building surface is, the straighter your planes will be. This is not the place to pinch pennies. Rather than trying to shim your current surface, go for something new. I like the idea of the granite top. Let's face it, it'll NEVER warp and will last forever. Lowe's, Home Depot and places like that often do have "damaged" doors. Also look at salvage and discount yards. IMO the building surface should be flat within 1/16" TIR from end to end and less than 1/32" TIR front to back for a 4' x 8' surface.

My current building table is an old solid oak library table. I topped it with a solid core door I got from a salvage yard. Dead nuts flat. I put down ceiling tile on that and finished it off with 3/32" cork. Both the ceiling tike and cork are easily replaceable when too torn up to build on.

Whatever you use, be sure it's WELL supported. I know some use sawhorses, but I prefer more stable mountings.

Dr.1