Earlier quoted context omitted.
Yes, AdBlock is more efficient because it doesn't load the ads at all while Chrome/greasemonkey load and hide them. No, I don't think it can be more efficient because chrome extensions like greasemonkey scripts cannot modify the page before it is loaded. I may be wrong, I had only a cursory look at the technology.
What about "@run-at document-start"? http://code.google.com/p/chromium/issues/detail?id=4478#c5
You're right, I don't know if this is enough (do you get hooks upon loading each resource, etc) but they are definitely working to implement this properly.