Im not saying a bov is the direct reason for running on 5. However a bov WILL make it run very rich for short times during gear changes. A very rich condition can actually make a car run lean because all the fuel can form into droplets that do not burn right. And so the little part that are still burnable will be to little to make a good AFR. This may again lead to detonation. Maybe not enough to do real damage right away, but if it happens enough times it will. And that would make it run bad. In addition he says the car has ran rich since the partswap and that do not help either. See?
Now this might not help his real problem right away but may help to avoid it form happen again when its fixed again.