Knowing your insulin sensitivity can be interesting, even helpful, but will not take the guessing out.
For instance, let us say you have low sensitivity. Well, many would naturally react by lessening CHO in your diet, as makes good sense. HOWEVER, many recent studies show that certain fats, along with their specific timing of intake, can alter insulin sensitivity in a negative way, perhaps worsening the problem.
IMO: Knowing your body through experimentation is more helpful than any test (at least, in this regard).