The 720 has a turbo-boost feature that will up a single core's clockspeed to 2.8GHz for single-threaded applications, if needed.
That being said, the i7 core is more efficient than other processors, and you don't literally need 2.8GHz, even if the game that you're taking it from says it on the box. Especially with a notebook, you're going to run into a GPU limit (depending on the game) before striking a CPU limit. What game is this for?