Vega is already drawing near 300W, and is so high up on the voltage/frequency curve that even a measly 5-10% gain in core clock can easily cost over 100W more.

Here, AMD is once again (see last year's Polaris) a victim of the inferior GlobalFoundries 14nm LPP process. TSMC 16nm would have been much better in perf/W, but sadly a very restrictive wafer supply agreement locks AMD to GloFo for the time being.