AMD says 650w for a vega 56. I think you'll be OK if you run everything at stock. Just check to see if there are 2 8pin or 6+2 pin pci-e connectors to power the card (there should be according to specs for the gold).
You should just about be ok, especially if you follow the undervolting guide in the Vega thread. Make sure you power the card with two seperate pci-e leads and not a single lead with the pair of daisy chained connectors. That is a good choice of psu and Corsair's SF units are argueably the best SFF psu's available at the moment. It's a pity they are so difficult to get at the moment.