As all surfaces have microscopic gaps or defects, basically all you're trying to do with the as2 is to fill in the microscopic gaps in the HS surface and core surface so the two can mate better. I suppose if both surfaces were perfectly flat (no microscopic defects), like glass or something, you wouldn't need thermal grease. So to answer your question, only a minimal amount is needed. Thats why lapping is so popular, because you're in effect doing the same thing (reducing the number of microscopic defects).
Most of the time, if you put a little too much thermal grease on, when the two surfaces mate tightly, the extra that is not needed will be squeezed out the sides.. no big deal. If you put way too much on, then you might have a problem where the HS and core are not touching enough to dissipate enough heat because they are on a thin layer of thermal grease...possibility for big problems. I don't think this happens very often though.
I think only a small amount will do the trick, like the size of a water drop or something along those lines... and spread around really well.
I wouldn't worry about it too much, but since I wouldn't want to feel responsible if you went on my advice alone... get a few other opinions before trying it.
Didn't mean for this post to be so long... got a little carried away. Sorry ;D
