It is based on observations from the tail I made. I got about 25% improvement in FE over the EPA rating numbers even with a tail on a sedan, you may have better aero with the tail on the hatchback. It is somewhat hard to tell what was the tail and what was driving technique but I think 25% is accurate for the tail only. I could probably do an additional 20 to 30% over EPA by doing EOC and slow driving.
Foam and fiberglass will make a nice looking part but the time to build will be very high. I have worked in the boatbuilding industry and build boats as a hobby. The only time I would build with foam and glass is to make a plug for a mold. In order to get a good finish and lightweight fiberglass part, it is best to lay up fiberglass in a mold, vacuum bag it and pull the part out.
I generally prefer to use chine construction with plywood in stitch and glue construction method for boats. It is faster and easier to get to a finished hull. But that does not give you the nice curves as you can get in foam/fiberglass.
I am still up in the air about whether to use plywood or coroplast for my next tail. coroplast is really an amazing material for the weight and cost, I probably should stick with coroplast.
The program Hull Designer can be used to get cut out grids for panels from a 3D design that you lay out. Then attach the panels at their edges and the 3D design appears.
Carlson Design Plotter/Cutters
This software could be used to make a tail with multiple panels that is more rounded than the somewhat ugly four sided tail that I built.