I don't know that I agree with the poster above with respect to:
"a top school would require you to study a bunch of mathematics to gain a certain level of rigor of your thinking"
Most good schools require only two courses in "pure theory": first an introduction to discrete mathematics, where you learn about logic and how to prove stuff, a smattering of number theory/crypto, graph theory, probability, and automata theory; second an algorithms course that teaches you to analyse and construct algorithms according to certain design methods. Together this is really only a year of mathematics, and both courses are quite fun.
Note that discrete mathematics as taught by a good CS program is NOTHING like the math you learned in high school or elementary school, it teaches you to think creatively about mathematical structures and then justify that creativity with logic.
If you're already a developer, you'll probably have a bunch of latent structures hanging around in your brain that will let you have an easy/fun time with the material in these two courses. I suggest that you study discrete mathematics and algorithms in depth, to the level of some decent undergrad course on each. Learning this material will give your thinking the sort of quantitative/logical "edge" you might want -- these two courses contain the basic mathematical tools common to much of computer science. I do agree with the post above that they are not the most practical things in terms of everyday engineering, though.
Addendum: if you find that you really enjoy the mathematical angle and you think machine learning is cool, also learn linear algebra, because a bunch of machine learning theory and implementation relies on linear algebraic algorithms and concepts. Linear algebra actually would be practical to know if you ever do anything with data analysis.
Even at good schools (in the US, anyway) the calc up to multivar that is required isn't done with what I would call good proofs. They'll generally do epsilon-delta definitions of the limit and a few other fun things, but will fall far short of a truly rigorous development of the calculus. Same goes for many introductions to linear algebra and stats with calc courses. Some math departments use linear algebra to introduce proofs and creative mathematical thinking, though, so this isn't always the case.
So if someone is looking to develop how they think in mathematics and they already have a good programming background, they are much better served by taking a discrete math course than any of those sorts of math courses. If they then want to learn calculus, I would recommend self-study from Spivak's "Calculus" book which is a fun and elementary but rigorous treatment of the subject. You get to see not just how calculus works, but why it works, which is sorely lacking in most undergrad calc courses, even up to multivar.
If someone wants to develop their mathematical thinking and is good at programming descrete math isn't going to help them much because they have probably already seen much of it. They need to be exposed to high level calc and linear algebra to develop their mathematical thinking.
Subjects like that give you a firm grounding in the area of computer science but you could study them for ages especially on your own. These subjects are also not very practical in terms of everyday engineering and on top of that before you even get to that level, a top school would require you to study a bunch of mathematics to gain a certain level of rigor of your thinking. My guess is that this is hard to do on your own.
On the other hand, since you have a related degree already and since you know programming, I would heartily recommend you to study on your own combining two approaches: bottom-up to learn the basics and top-down to immediately start increasing your market value and to start opening doors and to keep yourself motivated to go forward with the more tedious study of the basics.
E.g. say you are interested in distributed systems - you can start reading up on them (top-down) while digressing here and there to learn some of the basics.
Make use of online resources like Coursera for the necessary basics (bottom-up): at least some maths heavy on proofs [1], at least some complexity theory, at least some graph theory, ....
I think a purely bottom-up approach (going to a university) could be a waste of time and money in your case. But you need to emulate it a bit because a purely top-down approach could be too superficial and wouldn't teach you some of the more rigorous thinking you might need.
Whatever you do and learn try to do it in-depth. Superficial knowledge of many things will not benefit you much in the long term while in-depth study of many computer science fields will likely lead you through a series of small enlightenments.
[1] My professor of linear algebra used to insist that we as computer scientists have to know all the proofs of the theorems we were learning - in contrast to mathematicians learning the same subject - simply because we have to know how things are done to the last detail and we have to develop that kind of thinking.