STEP is taken after A-levels so does not really help beforehand. A good performance in the MAT might for some universities but honestly if the C is immovable you may be better off just taking a gap year, otherwise you could end up at a university considerably below your paygrade. If you walk out of A-levels with A*A*A and knowing you can perform well in STEP, you'd be in a great position as a post-qual applicant and would definitely get into at least one excellent course.
Whether Cambridge would theoretically accept someone on results day (who would have probably been predicted at least A*AA and achieved an offer for A*A*A + 1, 1) on A*A*C and say S, S is an interesting question (and a non-trivial one to answer) but one that I don't think they're faced with very often.