I wonder how unity is doing the incredible city demo and the fact they played it on a mobile as well? They talked a lot about streaming but even then the LOD was incredible with no popping.
Because they've gone into the core of the engine and rebuilt the way objects are handled concentrating solely on performance: specifically they've reduced everything to primitive variables in a tightly packed fast iterating array which speeds up CPU operations. Combined with a render culling algorithm and asset streaming they can see these incredible results. The downside is that going from object oriented programming to a data oriented approach is a bit of a headache.
Unity have the advantage that they're only trying to build an engine rather than a game. CIG are basically stuck with what they've got, to go into the core of the engine and implement the same system would take years and then they'd have to throw out all the code they've already built for the game.