I think it has to do more with the quality setting on the bump tab of the material editor for the displacement rather than polygon count.  You can apply displacement to primitives and rocks (very low polygon count!) and still get good looking displacment.  I use at least a setting of 2 for quality.  Render time is increased, but if you want quality, you have to spend time!  You can also bake the displacement, which saves on render time, but increase the polygon count...

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