After much speculation and comparison (side by side at the dealership), I have decided that the 00-03 SSEi tails are in fact darker looking than the GXP tails. The major differencesbeing that the GXP tails don't have the black horizontal lines and that they have no amber lenses.
These pictures aren't from the same angle, but they show the difference:
If you click on the lower picture, you get the full res (1600x1200) picture and can see the tails a bit better. I don't think I have a straight on rear shot. I'll see what I can shoot tomorrow if it'* nice out.
I was considering purchasing the GXP tails & center, but after really looking at them, I don't like the look on a black car as much as the way I like my tails. I'm just going to work at clearing out the amber sections to get them to look the way I want.