4930k is a bit less value for your dollar but it's the cheapest (and best value) six-core option, so that gives it some extra attraction right there.


But yeah, stock coolers require good clean cases to function even remotely well at high CPU loads. They blow air inward and it exits the bottom of the heatsink (preferably) and expands radially from there. If you were to put a piece of paper under the bottom of the heatsink (think of dust as a similar obstruction) you would probably lose half of your airflow over the fins right there.

The 10% speed increase you see is due to the fact that you're not hitting the thermal limits so the CPU allows itself to clock slightly higher.