Vega is ... hot. In stock it takes about 400-450W in dual, 300-350W in ETH-only mode at about 33MH/s. And it's throttling like hell. It's very good in dual mining mode, but it's too hot.
Have you messed with undervolting?
Usually I don't change anything, just use stock clocks/settings. But for this card I will have to change some Wattman settings due to hard throttling, but I found no Wattman. I spent a few days to prepare my tools and code for Vega, but today I finally noticed it: no Wattman for Vega at all. May be because I use Windows 7 instead of Windows 10, I will check it (though it's strange anyway). Also I don't see "gaming mode" button in drivers. ADL does not work too, so I even cannot set 100% fan speed to prevent throttling. AB does not work too. Weird.
Probably just due to Win7 for some reason. Wattman is there, in gaming mode in Win10 anyway. Gaming mode doesn't do anything now except get you wattman. You can also use wattool. Memory speed gets set slow if you set all pstates, so just set last one. Bugs probably will be fixed upon RX Vega release.