No need to add more geometry to get a smooth result. The mesh certainly looks dense enough. You simply have to modify the crease angle of the model inside kerkythea. Right-click the model from the left hand list, in the popup menu focus on "smooth" and from the submenu select your desired angle. In this case the "smooth" option works. To return to the above look after setting a crease angle select "flat". Simple as that.
From what I read in the thread below I thought I'd give you another tip. The above image doesn't really require an infinite plane. Use a groundplane and resize it to match the camera's FOV and/or required ground reflection on the object. This can reduce rendertime a lot based on the render method especially with photonmapping (both photon mapping, irradience caching and the rendering time). Just keep in mind that using an infinite plane is expensive in computational terms.
Oh yeah, on closer inspection of the model the sharp edges do seem to be close or over 90 degrees which will render the above method useless so you may have to do another subdevision or smoothing operator (Catmull Clark or Doo Sabin method). This can also be done inside KT from the menu but may not always produce the desired result because it's a simple loop subdevision. (right click => "Modelling" => "Loop Subdevision") Also keep in mind that crease angle will not smooth the visible edges of your model. Optimally for this model you'd need adaptive meshing. More geometry on the sharp borders and fewer on the surfaces within the same plane. But ultimately there is no penalty for the amount of geometry your scene has besides rendering time. The described methods are only for optimisation.