I think success is so subjective to each person which is what makes the comparison of grades so difficult but, as a rough guide my English teacher has told me that on average, between mocks and real exams, people tend to go up about 1-2 grades. Also, I think there's so much more pressure on real exams so I think that drives you for extra progression as well, I know it does for me, anyway. Hope this helps, good luck!