From what I can tell, Essence comes mostly from Crystal Crags and Shards. The Shards in the Imperium Wildlands are just too far away to have an effect on the settle-able location. On the other hand, in the Curgen's Tomb Wildland, once cleared, you can find a 423 location in the middle and, if they are not blocked, some 422 locations on the outer edges.
In other words, it depends entirely on what resources in the Wildland exist and how they are placed, if the city-locations have Essence or not.
I would still be in favor of changing the placement of the Shards in Imperium, though, as I doubt that Curgen would have build his capital-city on such a "bad" spot. 
