The core clock speed is your basement, and the boost speed is your ceiling. Your GPU will not go below your core clock speed and supposedly won't go above your turbo boost clock speed. With Turbo Boost 2.0, the variability between whether your GPU runs closer to the basement or ceiling depends on your GPU temperatures as set by your "Temp Limit" variable. Below your Temp Limit, you'll see higher turbo boost clocks, at or above your Temp Limit, your GPU will run at its core clock speed. People regularly see Turbo Boosts well above the turbo boost value listed in GPU-Z.
In order to overclock with Turbo Boost 2.0, you have several variables you can tweak, but temperature is the all important one. You can raise your Temp Limit from its default 80c, but it's good to also increase your fan speeds to make sure that you stay below your new temperature target. Raising your voltage is an option, but it often is counterproductive as it is the quickest way to increase your GPU heat.
Most overclockers have the best results by increasing their power limit, fan speeds, temp limit, and core clock offset. Then you check your "Reasons" variables (check Afterburner, PrecisionX, etc.) to see which one is limiting your overclock (1 means "limit reached"). If you have the headroom, increase your voltage if you wish, but in the end Turbo Boost will still ultimately control your voltages as well.