The one thing to consider about external bricks is they can go ~10% higher than rated wattage before things start to fail. If you plan to aggressively push the 500W limit on the Pico, then odds are without ample overhead you may blow one or both of your bricks. 280 X 2 = 560W + 10% = 616W peak. This is likely enough overhead to manage large momentary spikes without losing any of your hardware.