Can I advance an agenda over it's scoring cost?
Yes, you can. From the rulebook, p. 14


  Agendas can always be advanced while installed. Cards other than
  agendas can only be advanced if their text box allows it. There is no
  limit to the number of times a card can be advanced.


and 


  Scoring Agendas– When the number of advancement tokens on an agenda is
  equal to or higher than its advancement requirement, the agenda is
  fully advanced and the Corporation can score it