Beatrush underpanel already mentioned is a real boon to the Fit's stability at speed and improves mpgs as well. Here's a link to my post with pics:
Beatrush UnderPanel
You may want to read more posts in that thread, it's got a lot of good information.
I am VERY interested in a rear diffuser that will smooth out airflow and reduce the 'parachute effect' of the rear bumper.
One problem with Fits may be that the two trim levels have very different rear finishes, the Sport model having a rear lip at the bottom edge of the bumper. If that turns out to be the case, I would like to cast one vote for you starting with the Sport model.
The other thing about the back end is the jacking point bolted onto the bottom of the car. I'm not sure I'd want the whole back end to be as low as that! There is a Noblesse rear diffuser that mates up to the JDM Noblesse rear bumper; I'm 99% certain that the jacking point needs to be removed to install that piece.