I'm surprised that this simplistic idea of Michael Gove has surfaced again. Of course teachers need to know what they're talking about, but I suspect that, once we are assured that a teacher has the necessary basic knowledge, there is little correlation between academic performance and teaching effectiveness. In fact, I suspect that if the teacher himself/herself has had to struggle to grasp the basics then he/she will have more sympathy with and understanding of the difficulties of a pupil who faces similar struggles. Able mathematicians and physicists in particular can easily zoom off into the stratosphere leaving ordinary mortals behind,
Many teachers, myself included, claim that we did some of our most successful teaching in our early years when we were often literally just a page ahead of the students. If you've had to bash your brains the night before to understand a concept you tend to be good at explaining it the following morning. Later, when a concept is thoroughly familiar, it is often hard to remember why it is difficult to understand.
In my view good teachers have four qualities: they have sufficient basic knowledge of the subject for the level they are teaching, enthusiasm for it, are good communicators and, above all, have respect for the people they are teaching. Of these only the first is learnt and assessed at university.
The teacher who is bored by the subject will soon bore the pupils. Many teachers are masters of their subjects but poor communicators. This was borne out to me several times when I attended evening classes on "car maintenance," only to be left behind by a skilled mechanic who was master of all the intricacies of engines but had little idea how to break down his subject into "learnable chunks." Teachers who do not respect their pupils, be they infants, potentially rowdy adolescents or old age pensioners, will soon be sussed out and condemned to failure, however great their knowledge.
So it is probably far better to be taught by an enthusiast with a third than the first class honours graduate who feels his superior qualifications entitle him to higher things.