The Impala and Grand Prix are based on the same GM platform, so they're probably going to have almost the same space. I've driven both, and they are practically clones except for the external and internal styling. It's like comparing a Chevy Equinox with the Pontiac Torrent
All the car companies just clone thier platforms for the diffrent brands anymore ... GMs epsilon platform is used for the G6, Aura, Malibu etc ...