Technical question: sprites vs. framerate

Just wondering: Does Torque draw only the layer that is seen, or does it draw the bottom layer, and then redraw the next layer on top? (does it take up time by redrawing stuff over other stuff) I'm making a scrolling tile map. A grass layer fills the whole screen, and a lava layer on top of that, and rocks and trees, and on-and-on, and about 20 animated "people" and partical effects. I anticipate using enough sprites and tiles to fill the screen 4 times on any given frame. It seems there was a test done with 4000 static sprites with no challenges. I'll only be using 3000 (but they'll be big enough to fill the screen a few times)

Question 1: Do the tiles that never show need to be erased for the sake of frame rate? I know Torque does some optimizing, but can it handle drawing to the screen 4x over?

Question 2: Some of my sprites have lots of alpha channel. Is it better to cut these up into smaller chunks and reduce the amount of alpha channel for the sake of frame rate?

The real question is "Do I need to take the time to erase stuff that doesn't show anyway, or do the Torque optimizers take care of things like this?"