Putting a heat sink on top will restrict the airflow over the pcb. The pcb has internal copper layers ('ground' is usually designed as a plane) which act as heat spreaders as well.
Do you say the other side of the pcb needs airflow too? I intended to force the airflow from the fans through the heatsinks only to maximize cooling. Do i have to rethink this?
The chips are designed to release heat out of the bottom, and putting a heatsink on the top of the
chips can restrict convection and cause the chips to overheat. The top of the board doesn't need additional air flow AFAIK. The fan on the heatsink should create enough airflow around the board that all you need on top is convection for the other parts. That is, don't cover it up, but no need for anything else.