Interesting... How often do you stack cargo to the roof? The only times I have had it impact cargo is when carrying something with very boxy dimensions like a washing machine. My Veloster has a pretty sloped rear hatch but I'm surprised it doesn't seem to impact space much, I guess because I rarely stack cargo higher than the top of the rear seat (which is still quite a bit more than one would normally put in a sedan variant - of one existed).
The way I see it, with a hatch, 80% of the cargo advantage over a sedan is the hatch vs. trunk opening size*, not the ceiling height. Look at a Prius for example... Very sloped rear glass, but massively useful cargo area.
*This opening size is a problem with some newer cars though.
