I think that students surpass their teachers in martial arts & in sports, where youth, speed, etc... is extremely important in those situations.
In aikido, it is different though! Timing is more important than speed, ki development is more important than strength, etc...
Therefore, I stick to my first statement. In AIKIDO, I don't think that a student can overcome a teacher of high rank. I DO think that a student can overcome another person(teacher or not) that is 4-5 ranks ahead. (EG. a yellow belt could surpass a shodan in 8 years if the yellow belt picks up and trains in a more positive manner than the shodan does in those same 8 years).
I hope that I am making sense. I know that there is no way I could ever have been at Bill Sosa Sensei's level. Even after his passing, I doubt that I will be at his level if I live to be 80! There would have been no way to be at his level had he been alive when I was 80. (Does this make any sense to anyone)