A small "spoiler" at the end of the hatch could help a tiny bit or hurt a tiny bit. Unfortunately, outside of a wind tunnel, you'd never be able to measure what effect it has.
FYI, the Chevy Volt aero team determined a small rise in the trailing edge of their hatch helped a very small amount.
See:
http://ecomodder.com/forum/showthrea...ures-4629.html