OK folks, so far I have two votes in.
basjoos voted for the Kammback (along the lines of MightyMiras) , though his post got lost in the crash.
And I have one from diesel john voting for a splitter on the front airdam.
While I agree that a splitter may be beneficial, I don't think that it falls into the category of "low-hanging fruit" at this point, and that's what I'm shooting for with my dwindling supply of ABS.
I'm inclined to go the Kammback route because it seems the wake of my car would be one of the biggest contributors to drag at this point, but the other biggest contributor to drag is the front wheels (~13% if you believe the Volvo graph).
So, what do the rest of you think???