As mentioned, without riser mount it co-witnesses 100%. When I ordered mine, I also got the smallest mount I could find at that time, and it puts it so high that it doesn't co-witness anymore. Just the other day, I came across another mount that looked smaller than the one I originally got, so I'm hoping it will provide a tiny bit of co-witness (may be less than a 3rd though). I'm probably wrong, but if does sit lower and provides some co-witness, I'll update this post.
Otherwise I'm happy with mine. In fact, I just ordered two more. One for the Bren (since the EOTech is so darn heavy) and one for the vz 58. (Got an RS mount on the way).