It already does. The FMC gives you an optimum cruise altitude and guidance altitudes for heavy turbulence are generally 4000 feet or more below that optimim. What you are proposing is starting to sound more like a technical impossibility: a device that takes radar returns, calculates the intensity of the turbulence in a rapidly changing weather environment then fires off a series of predictions of what your margins will be. Notwithstanding the technical difficulties, you don't need a computer to tell you what speed to fly at in turbulence because the manufacturers have already told you what speed is best. You don't need an amber advisory because you can see what speed you're doing on yout instruments and compare it to the manufacturers published turbulence speed. If you need a flashy advisory to remind you of your turbulence speed you shouldn't be in the flight deck.
Even if such a device existed, there are up and down draughts throughout cells with different turbulence associated with each. Each encounter will be different and poses the potential to see you chasing one speed target only to find it's a completely different one 5 seconds later. You'd be more likely to get into a pilot induced oscillation from following that, which will do you more harm than good.