The extra shim behind each of the spider gears helped bring back center diff performance. Initial testing with battery pack in buggy felt like I had drag brake on 50% or so. After a lap at mild speeds I let 'er have it! My buggy was breathing again!
As for how long the gears will last in their worn condition, I don't know. This is my first time in two years that I am experiencing grinding noises, and experimenting with shimming on the center diff.