20000 polys and over is only too many if it lags the computer. If it works and it still allows a broad range of your customer base, go for it. However, it depends on what else is in the game, particles and such. So again, it depends on the game.

LOD would most certainly help. Getting rid of faces you can't even see b/c they are so far away will free up the framerate.

As for the new splinter cell, google it. It was on TV when I saw it.