My burmer is 8x my disc are 8x so what is the difference between setting burn speed at 8x or max?.
There is a difference. Some good brands can be burnt at a higher speed than the certified. Depends on the media, burner and software of course.
As far as I know, max will determine the maximum appropriate speed for whatever disc you are using.
None, as your writer can't write faster. You can set 16x if you like as well, no difference.