Hi,
I did consider this option myself, before buying the Noctua. It does have some good reviews and from what I have read about it should bring your temps down quite a bit. I have also read that it can be a little tricky getting it to fit properly so be careful and read the instruction manual carefully prior to installing.
Good luck and let us know how you get on.
EDIT: You changed your post! To answer your questions:
I believe that it is designed to draw cooler air, from outside the case, through the radiator to promote better cooling. So I think you should leave it as it is. (But I'm sure someone will correct me if I'm wrong)
The thermal grease that is applied to this cooler is OK so you should have no worries there