I see references online for "Hawking has never been interested in how high his IQ is, but it has been estimated to be over 160.", which puts that as a lower bound, but not at 160. My IQ as tested in elementary school was over 160, so I suppose that doesn't seem super high to me. I'd guess Hawking's at much higher than 160. Without direct testing comparisons, there is a lot of "estimating" that goes on by various people, but pretty much anything over 150 is going to start getting into the realm where it tough for most people differentiate.