Power-off storage temperature is a problem for MLC NAND, but when powered on the NAND cells need to reach a high temperature to operate properly. If you are cooling the flash chips with a heatsink (rather than cooling the controller) you will be forcing the device to dump power into the cells to heat them to a temperature where they work properly.

3D NAND hits its best program time and raw bit error rate at about 70C.

Edit: See data table on page 27. Retention is directly proportional to device active temperature, i.e. higher cell temperature during programming leads to higher retention. https://www.jedec.org/sites/default/files/Alvin_Cox%20%5bCom...