How far light penetrates into the canopy depends on what wavelength is hitting it. Green penetrates the most, and some of it even makes it to the bottom and bounces around. So if that's true (and it is) what kind of bulb you buy will affect how far light penetrates. All HID have a fairly large and varied spectrum. Some are more red, some are more blue, some have a fairly even (or full) spectrum, and some have spikes in certain colors.

Most of your light will bounce off the plants. That's why it's important to use reflective material or white paint on the walls, and to keep leaves from shading buds. And do like Rusty says and keep your plants gently rustling in a breeze to make them strong.