My guess is that the designers were channeling (no pun intended, and
not much of one achieved) one of the classic hotrod looks:  a "chopped"
roofline.

The surprising thing about that family of Chryslers, to me, is that
they're no worse than about midpack among large sedans when it comes to
headroom.

Reviewers differ on vision to the rear -- I suspect that driver height
(seldom reported in reviews) and preferred seat position have a lot to
do with this.  The Magnum, with its sloping roofline and a rear window
effect that one reviewer likened to a mail slot,  seems to get
criticized a lot more than the 300C in this regard.   At least nobody
will mistake it for a minivan.

My highway experience with the Hemi versions is that the sightline
problems, real or illusory, don't seem to dissuade their owners from
passing me.

I guess what the original poster will really have to do is saddle up
and go for a ride -- again, how much you can see in which directions is
probably a pretty personal thing.  Iterate with the various drivers in
the family.

--Joe

And I'd add that when I had a Magnum rental car, I was surprised at how
good the rear visibility was, given how small the windows are. They're
small, but they're well-placed. My wife's 93 Vision TSi has the opposite
styling disease (huge glass area / beltline that sweeps up toward the
trunk), but rearward visibility isn't really any better because of the
high trunk.

Uh-huh. Because all those cars without super-high beltlines have such
cruddy side impact protection.