There is an air-gap inside the chip packaging, so heatsinks on top will not provide any benefit, and according to Avalon will actually reduce the cooling performace. There is a thermal pad underneath the chip, which gets soldered to the PCB along with the contacts and then vias conduct the heat through the board to the heatsink on the opposite site. There's quite a reasonable discussion BkkCoin's Klondike thread about this.

We definitely plan on applying thermal paste correctly