If you look at my screenshots of the finished tunnel, you'll notice that it gets darker in the middle as you get deeper into the tunnel. This has a number of effects - dropping the framerate, yes, but it also looks pretty cool and hides the ugly moiré effect in the distance.
How you decide to implement the effect is up to you. You have a depth table, so you could use that to scale the brightness of each pixel. However, I prefer to build a new surface in memory, set it to black and edit the alpha levels, so I can just blit it over the final image (shifted around and clipped to allow for "looking around") which takes advantage of the hardware blitting offered by your video card.