The prediction does matter as many universities want predicted A*s for some of their courses, and any predicted A*s (depending on the course) will help your application on an academic basis.
Every school and every department will differ for this. Some departments at some schools will give you an automatic A* prediction as long as you get 90% UMS and are not horrendously failing the beginning of A2. Some departments will take into account how you are working in A2 more, and so you may get away with 8x% as long as you are performing well in tests, and were before your exams (such a system will be in your favour if you did not do as well as you wanted at AS or are planning to resit modules, but will work against you if you appear to be struggling with the A2 content.) Some departments at some schools will compare your AS UMS to the UMS of those in the previous year(s) who achieved A*s at A2 to see if, statistically, you have a realistic chance.
All in all - there are different ways, and the only people that can tell you for sure are your subject teachers/heads of dept. Good luck!