The problem is that it doesn't matter if the top-end level of the map is 3 levels higher than you or 30 levels higher than you, you get the same XP per kill. There's no reason to have to jump into high-level maps for power leveling, all you have to do is make sure you jump to the next campaign level as soon as your current level is 2 or less below the current area's maximum (which, "coincidentally", is now the new minimum character level for the next area).

So unless you're simply trying to leech off of farming groups, you'll do just fine playing the game as it is currently designed, presuming you jump to campaigns as soon as you can. An experienced player should have no problems playing through the campaigns at intended level minus 2.

So, just pick a good farming location in each campaign, and grind away!