I was evaluating simplygon as well using the HLOD system… In short : not useful at all. I have a better suggestion for you. Instead of using HLOD use World Composition and put clutter into small levels. That helped me a lot on my scene. HLOD batches actors and replaces them with a single object, however, that object will increase your texture pool, because it creates that atlas…
I didnt like the system at all, the artifacts when combining actors was awfull… The Tris count you see is average. So instead of of having an average tris count of 10k you have a single object with a tris count of 230k… Now you could reduce the polycount with simplygon but the quality is just blah…