Hi there,
You're right - both have applicability - no doubt about that. But it's difficult to bifurcate the impact of that exam versus things you learned in grade school. Loved your comment about cryptography
. I think the main thing GMAT is trying to test is your ability to think under pressure. You can score a 750 and not know anything about number theory as long as you understand the strategy and have a solid grasp on most of the other concepts. So while you need to know most of the topics tested, you definitely don't need to know them all either. Going from a 650 to a 750 is as much as about strategy as it is just raw knowledge.
I see what you're trying to do here, however. Things like algebra, linear equations, percents, powers, stats, etc. will all play into your MBA experience. Understanding these concepts will be important no matter what you choose to do. My point in saying it wouldn't be incredibly applicable was to say these are fairly basic concepts. They might be things you're no longer too familiar with (it's probably been a while) and while getting up to speed during prep is great, it won't be the advanced frameworks you need to rock the classroom. For that, there are a lot of pre-MBA prep programs out there (e.g. MBA Math) that focuses on the math concepts you'll need during bschool.
That said, practice makes perfect. And if prepping for a bit more means a better score AND you know the basics inside and out - then that'll be a great foundation. Also, really focus during this last month on test strategy. For example, if you absolutely cannot get an advanced topic, you'll probably be better off learning to recognize it and skip than you are trying to understand the concept. Because the more you study a topic, the more you put yourself into a trap when you're taking the test thinking, "oh, I studied this, I should know this, so I'll spend some time here..." When what you should have done was just move on immediately. Especially because super advanced concepts don't show up with that much frequency.
Let me caveat this by saying we are not a GMAT prep company nor do we provide any test prep services. The advice above is meant in good faith.
I hope that helps!
Bhavik