I currently have 4 sticks running at 16Mhz at full hash of 447Mh/s with no changes to the heatsink. (I didn't even remove it to do the soldering.)

current temps are around 37deg C with ambient around 16degC. my 13.5Mh/s units are around 32degC. I have a 23cm desk fan blowing air across my miners.

it seems to work well.

so no you don't have to modify the heatsink.