I actually believe the biggest reason for the none availability of offline update files is due to the business model. Less offline circulating, less chances of piracy (not wholly applicable... i know many rigs which are never on the network so no update.). Anyway who would want to manuall install files 2-3 times day just to keep updated cos if you missed the update, then you are exposed already.
Of top of that i was made to understand that the files are actually verified and must be installed in a certain manner into the system to avoid corruption of the checksum( could be bullshit though...). I must admit i never got a clean answer even from the support guys.

Hi,
see www.av-comparatives.org
Look under the comparatives adn you'll get 2 sets of comparatives...the full info on detection is there. Speed is relative...

hope this helps...