For what its worth, this weekend I went down the local rock store and got some grinding grit, and picked up some glass. Time to do a little testing.
I did a small test piece of glass with 200 grit and a piece with a grit that was marked as 400-600f. To be honest, there wasn't much difference in the glass, but there was a huge difference in the size of the grinding compound. It did seem like the 400-600 was a little sharper, maybe a little brighter, but the 200 seemed a little less prone to the hot spot.
So, I thought, what the heck lets rub some 200 on the plastic and see what happens. It was magic. In ten minutes time I had a usable ground plastic-glass. At 18 to 24 inches I could easily see the entire 16" screen for composition.
I then did a 200 grit on real glass. It was sharper, clearer, and maybe a tick brighter. But not enough better in my mind to make up for the "break factor" of glass.
I may try a 400-600 and see if its better. But for now my 200 grit plastic looks pretty good.