It looks like we need to factorize qMRMLVolumeThresholdWidget with qMRMLWindowLevelWidget.
Maybe create qMRMLVolumeDisplayWidget as a superclass for both of them.
Example:
class qMRMLScalarVolumeDisplayWidget{
public:
vtkMRMLScalarVolumeNode mrmlVolumeNode()const;
vtkMRMLScalarVolumeDisplayNode mrmlDisplayNode()const;
void setMRMLVolumeNode(vtkMRMLScalarVolumeNode volumeNode);
void setMRMLVolumeDisplayNode(vtkMRMLScalarVolumeDisplayNode node);
...
protected:
virtual void updateRangeForVolumeDisplayNode(...)
// to be reimplemented
virtual void updateWidgetFromMRMLVolumeNode();

// Pimpl can also have utility functions
class Private{
virtual void updateSingleStep(double min, double max);
...
};
};