This is a question that every further student ponders at some point. Ultimately I do recommend what you are suggesting but understand that it is a risk. The logic is straight-forward - feeling that you're in a better position during the exam when you have an extra module up your sleeve. e.g. on an exam, you might feel that module X has slightly easier questions than module Y.
Just keep in mind that your teachers/peers are less likely to help you out with matrices. Also, you are less likely to "touch all bases" in the module since you haven't had a SAC on it/no dedicated class time, etc. But if you are determined to self-study matrices then it should be no big deal. In any case, I would aim to learn both business maths and matrices (with the emphasis on matrices, but don't completely neglect BM).