Workflow for adding high poly features to a largely flat landscape

Lets say I want to add highly subdivided features to a largely flat landscape.

Out of curiosity, I guess the explicit questions are:

  1. Is there an algorithm that culls vertices if a set of faces that make up a larger flat face?
  2. What is the approximate performance gain?
  3. If so, in which renderers is it implemented for? (view_solid,view_graphics card,BI,cycles,etc)

I’m guessing it is still a good idea to avoid adding simple subdivision to a face needlessly.

You could sculpt the features in Sculpt mode and dynamic topology on (Enable Dyntopo button).
If you want to reduce face count, use the Decimate modifier…