All throughout the arc Oda has portrayed Impel Down as both a literal and metaphorical hell. Yes the guards dress like Satan and the levels correlate with Dante's Inferno, but it's also the ultimate expression of the World Government's oppression. If piracy is how freedom is symbolized within the series, Impel Down is the place that those pirates go to have all their rights and freedoms systematically stripped away. It isn't enough for the World Government to kill them, they have to crush their spirit and kill their dreams, with the very worst offenders being erased from history altogether.

Level 5.5 is Impel Down's antithesis, the Garden of Eden hidden amongst the flames of hell. It's here that certain prisoners are able to claw back their freedom, and I think it's significant that Iva says that that they have to "travel down paths that don't exist" to manage it. It's Luffy's journey in microcosm, as he literally is searching for an uncharted treasure in order to become the freest man in the world.

I also think it's significant that the guards of Impel Down themselves are confused. They think that because the prisoners have escaped their control they have been spirited away to some deeper hell, when it's only after leaving the influence of the World Government's power that the Newkamas are truly able to find their freedom.