GPU Boost is an algorithm the GPU uses to clock up and down based on power draw, temp, voltage, and application. It's automatic between GPU and drivers. The colder the GPU is, it'll clock as high as it can for as long as it can if it has the power budget to do so. With the 3000 and 4000 series cards they are mostly voltage capped rather than temp capped. So trying to increase is rather fruitless unless you can get them real cold.