This shows several combinations of heatsinks (active, with a fan, and passive). We tested this by monitoring the temp. of the CPU (via the BMC), while the system was idle (for several minutes), then starting multiple instances (1 per core) of heat, running those for ~20 minutes, stopping heat, and waiting 'til the temp returned back to the starting temp.
We have two machines, that have two configurations of heatsinks (with and without a fan) and fans (that force air towards the CPU). The graph below shows that the 6 x circular fans (high RPM, with high yaw, attached in 3 rows of two fans) keeps the CPU cooler than the 2 x squirrel cage fans: it also shows that the passive heatsink keeps the CPU cooler than the heatsink with a fan attached (the fins on the active heatsink are substantially smaller).
This is a link to the xmgr image source file. This is a link to the postscript file used to generate the following .png