If you do decide to try something like that, I'd definitely be curious to see what happens.
That setting that brings Vega 56 down to 159.4W mentioned above. Does it match the GTX 1070 in performance, exceed it by a hair, fall short by a hair, etc...
And Vega 64, say similar things were done, does it match, or even still outdo the GTX 1080? Or, perhaps, slot between the 1070 Ti and the 1080? How much power does it use in that case? etc.
I can't imagine they were losing money even if they sold at MSRP (presumably to compete against their targets of the 1070 and the 1080). Of course, cryptomining threw all reality out the window. I can't recall, but I thought they were originally released to undercut the 1070 and 1080 prices as well (and the 1070Ti was released just to try to top the Vega 56 without infringing on 1080 territory, albeit it got a little awkward)
But you've piqued my curiosity - I don't have the resources to try the testing you suggested, but if the curiosity about this is nagging at your brain as much as it is mine, I'd love to see what results you get.
As a quick reference, this page
is what I quoted from about the "71.6% of the power gives 89% of the performance."