@ucbalumnus, we struggled over this, and eventually contacted the admissions office of an Ivy where my with is an alumna for advice and input. They, more than anything else, convinced us that it was better for her to get the IB diploma than to stress about losing her advantage in math. Incidentally, the time that the admissions office spent with us over this was mind-boggling, and really brought home the advantage of going to a school that will truly support you for life.
Anyway, I suspect that getting that IB diploma will pay dividends in the long run, even if you can’t fully quantify it, but again, the important thing is being in an environment where you will thrive. But OP, why not drop the philosophy (saving it for college) and take both sciences? My D is taking both HL physics and HL bio. Someone else mentioned the EE, which can serve as an incredible writing sample if and when you need one–not to mention that it will train you well for the sort of research you will have to do later in your academic life.