I quote A recent post by Cathar, designer of the cascade etc:

"Almost every single cooling device on the planet will perform better if the IHS is removed. The IHS is a hindrance to the removal of heat from the CPU die. It does nothing at all to help keep the CPU cool except in the very limited scenario of poorly designed and performing aluminium based heatsinks. Anyone remember the original stock all-aluminium P4 air-coolers? Anyone?"