I assume that with metal core, we are looking at very heavy PCBs. These offer a number of interesting differences to normal PCBs during reflow. Due to the mass of metal, it normally takes more energy to warm up the circuit boards in the oven, so more energy may be required at the early stage to heat up the board.
However, once it is heated up, it stores the hot for a longer time. Therefore, care has to be taken with heat sensitive components to ensure they are not damaged.
Generally, more energy at the start of the reflow profile will heat the PCB more quickly. Maybe you should use the last zone if you have enough reflow zones to start the cooling process. Reducing the temperature in the last zone to approx. 200 centigrade can help to start the cooling process.