I've searched the site, and found this:
"The lexan clamp will allow more pressure, but be careful. More than the 20 pounds AMD recommends, may result in damage to the cpu.. With the lexan clamp option, tighten the 4 thumbscrews in small increments in a criss cross pattern. "
found here:
http://www.aquastealth.com/install.html
It's no wonder that they recommend shims!
Consider replacing the mount with this:
http://www.caseetc.com/cgi-bin/caseetc/WC-423.html
or, even easier, get the springs. (anyone know where?)