Hello,
I'd like to know more about your extracurricular experiences, community involvement, leadership experiences, etc. But, if you have some good material there and you could get your GMAT score up another ~40 points, I do think you could be competitive at those top-tier schools. Your undergraduate work and your international work experience are great.
I don't think the Masters in Mathematical Finance will help you much in the MBA admissions process. Especially since you've already earned strong graded studying math and finance in undergrad, you don't have much more to prove there. In fact, such a degree could
possibly be a little bit of a negative, since you will have to explain why you earned that degree (which usually would take you into specialized financial engineering work or something similar), and now you want to add an MBA. You may come off as a little unfocused or as a "career student," which isn't necessarily a good thing in admissions officers eyes. Also, earning that degree and then applying to business school would start to put you toward the older end of the applicant pool, and that's not necessarily where you want to be these days.
Overall, I'd say focus on getting that GMAT score up a little bit first!
Scott