Here's some info to use for an edumacated guess:
According to:
http://www.tfd.chalmers.se/~lelo/rvad/reports/rva2002_gr02_underbody.pdf
Audi improved the Cd of the A4 with a patchwork of panels applied to the underbody. The sum difference was Cd -0.019
Assuming my continuous smooth under tray achieves at least that difference in Cd on the Blackflea, the rolling & drag spreadsheet tells me I should see a FE improvement of
4.6% @ 20 C @ 55 mph
EDIT: that's a slightly conservative estimate, since the Audi calcs apparently do not include the large front panel.