So I recently got my hands on Solidworks 2010 with the Flow Simulator add-in and I mocked up a *really* rough model of my Civic (extruded a side profile, then extrude-cut the front profile out of it). I'm still learning surfacing, so that will be a future project, but I figured this was a good starting point.
So, I put up several configurations. There was just a baseline run, then with a spoiler somewhat similar to what came on the Si models, then with a grillblock and spoiler, then with a kammback and spoiler, a couple types of rear extensions, and a 3.5 inch drop. Since the model is not super accurate it has a flat underbody by default.
I had it test for a few things: drag force (Z-component), lift (Y-component), velocity (it's in in/s since mph is not a choice), and pressure. Since it's in IPS mode, I set the airflow to 1200 in/s, which comes out close to 68 mph, which is a bit under the speed limit here in Florida, but that's about how fast I drive.
Believe it or not, the spoiler didn't increase drag very much and it reduced lift quite a bit. Lowering was far and away the most effective at reducing drag force.
I put all my results (spreadsheets and pics) in a zip, but it seems to be a bit large for the forum, so I made a mediafire account where they can be accessed.
results.zip
results.zip
Supposedly, those are the same. I don't really know how it works, but I figured better safe than sorry.