[christofoo]

I have a trunk mounted bike rack - pretty similar form factor to the hitch mount, just different attachment method to the car.

You definitely will get considerably better mileage with the bike on the tail than with the bike on top, even if you don't do a Kammback.

Are you sure you want the Kammback mounted on the rack? I think either car could have a Kammback mounted to the car as a separate item. For the hatchback, the bike probably won't even affect drag, and on the sedan it's probably a pretty small contribution, since it does stick out of the wake a little, but really not very much (the wake on sedans, excluding about the last 5 years of production, is a few inches above the trunk lid, I think). Of course the Kammback provides its own benefit in either case.

If you want it all on the rack you'd have to design it (with plenty of folks here ready to look at your ideas and give you pointers).