Occlusion is coming along. Today I think I finally have it to the point of it being visually clean (little to no object popping, no shadow popping) and mostly performance neutral when tracking low amounts of objects (occludees).
The problem with occlusion is it adds some CPU and a little GPU overhead, so it can actually reduce performance in the worse case. If you are outside and can see most of the world, then not much is hidden, so the overhead will be higher than the gains from not drawing a few things. If you go inside or have a wall in your face and 95% of the occludees are hidden, the you get some FPS gains. The more objects that are tracked, the more lopsided the best and worst cases become, so a careful balance has to be reached for what it worth occluding.