Otrebla- With the 9.7 sail, and maybe a larger fin (60+ cm), your Go 170 will plane quite early and easily. A formula board or the Apollo would be a little better, but the benefit might be too slight to be worth the money.
I think that a fast, displacement board would be better for sub-10 knot winds, especially on a lake where the winds are often gusty and inconsistent. Like JM says, a Serenity would be great. So would any longboard raceboard, like a Mistral Equipe or Starboard Phantom 380 (if you can get one).