The way I've played it is that you can turn while sprinting, but you can't ever move into the part of the board that is in your 180-degree back arc at the start of the activation, and you can't do any final adjustment to your direction when the sprint stops. This means that it is possible to do things like sprint around a corner that is in front of the model at the start of their activation. They could also end up facing the opposite direction that they started, but to do so they would have to sprint in a (small) half circle. Exactly how fast you can turn in such a case is of course not very well defined, but just knowing that it's not well defined would be helpful.
Ideally, it would be nice to get some official rulings on this, preferably with illustrations