While we're on this subject, I've been looking at what's being done with semi trailers--that may be related to driving an xB. It seems that FE is improved when the vertical side/corners are rounded, often with attached fairings. This has led me to wonder about the old station wagon deflectors that diverted air downward to keep the rear window clear. Would a similar deflector mounted vertically at the rear corners of the xB would help fill in behind. Sounds too ugly to contemplate. Interesting idea, though.

IMO a flat back (such as an 18-wheeler or Scion) is the worst scenario drag-wise.

Anything you can do to round the rear end is going to help aerodynamics at high speeds.