On the other hand, it is sometimes said (in folk circles, I'll admit, I haven't seen papers about this) that the good performance of Chinese students is related to having to learn a very difficult writing system, which acts as a cognitive "exercise" or stimulation.

So what's better to learn as a child, an easy to learn language or a really difficult one? Or maybe an easy one orally but a difficult writing system? I have no idea, but it's an interesting question indeed.