can tell you that we are recommending to our pro teams as well as pro-triathletes to run 21mm tires with both 808's and 404's, and NOT to run 18 or 19mm tires. We have seen no repeatable aero advantage in using 19 or even 18mm tires with these new rim shapes (though with the new 404, there was a slight tendency for increased drag with the 19mm tire, more here: http://www.zipp.com/...abid/60/Default.aspx), and feel that the increased ride harshness, reduced contact patch, increased rolling resistance and so on associated with narrower tires just can't really be justified on either of these rims.
Now, if you are running an H3, old Zipp 440, Campy Bora or Reynolds wheel, then we have definitely seen advantages in the wind tunnel to running 18 or 19mm tires over 21 or 22mm tires, and I would strongly recommend them for those applications from a pure speed perspective, but with the newer Zipp rim profiles, I can honestly say use a 21mm at a slightly lower pressure, and save yourself the added harshness of the high tire pressure as well as gaining back some improved handling and still maximizing aero benefits. These latest generation wheels have been fully designed around 21-22mm tires, so you can be confident that you are not sacrificing any performance with them.