The difference between a 770 and a 780/790 is mostly statistical. It is not a real difference that can be accurately measured.
The exam has +/- 30 to 40 points. So in a retake, you could well end up with a lower score.
Improvement at these levels is difficult. You are already performing well and leaving aside, matters of "luck", there doesn't appear that you could change things. Of course, you can practice selectively, but that is not a guarantee of any score.
Focusing on applications would be smarter. That will help you much more than spending time doing more practice.
Posted from my mobile device