Rendering with elements may add a little extra time, but not enough for me to figure out the difference - certainly not double the render time though :-)
Mind you, it was a simple scene for the tutorial - rendering a bigger scene now, so may see something then...
Of course there is the extra render of rendering the plane in this example - since you can split the scene into layers it would be great if you could output the imported 3d object in the the same render pass to a separate set of render elements, which would speed things up enormously (Matt - any chance of this in the future? :-)
In any event for an imported object that moves indepently of the scene, I'd want to render it out in a 3d prog, as Terragen doesn't do motion blur for animated objects yet.
Nuke is expensive, but other comp programs could handle the basic comp just as well, Blender or After Effects, though the relight tool is a little specialised.