File this one in the counterintuitive column. According to an article in Education Week, elementary and middle school math performance isn't really benefitted by having a teacher who majored in math. The study being reported on found that prior math experience (undergraduate major, previous career in something like accounting, etc.) didn't have much impact on the younger grades. By the time high school starts, though, the connection grows stronger.
I say that it's counterintuitive, but that's not exactly the case. As anyone who's ever been into a classroom knows, there's a lot more to teaching than knowing the content. Knowing the causes for the Civil War doesn't necessarily mean you'll be able to convey them well to a classroom full of hormonal teenagers. You also have to know how to teach.
That's the punch line in the whole push for "highly qualified teachers." In reality, the push is not toward highly qualified teachers, but rather toward people who are highly qualified in their field and who will become teachers. I'm not saying that's a bad thing in any way. However, we shouldn't confuse highly qualified mathematicians for high qualified math teachers. It may or may not help, but it's not enough on its own.