They are quite significant things. It seems, though I don't know the whole story, that if you're going to stand a chance you would have to take a year or two or three out first, to boost your application (get the grades; and be convincing that you are *much* better than your grades). I'm afraid the CSAS just doesn't make up that much difference (that is, the difference between your grades and the ones Cambridge would expect). This might be far too much of a hassle, and a bit of a waste of your time.
Perhaps best to aim elsewhere?
(NB: Only do what YOU want to do, ultimately, because it's very much your life!)