That fits exactly with the projections in the previous thread about power consumption. With GPU only, it barely fits in there. If the CPU were significantly active at the same time, the performance hit would be immediate and severe.
Also, since it's using 438W of a 450W maximum, the addition of bus-powered TB and USB devices (up to 75W total) could impact performance.
I addition, I'd like some real-world benchmarks (running the test for several hours) to see if it still holds up when the machine is warm. It's unclear if these benchmarks are from a cold start--in this case, the numbers could be quite different.
It's possible it can maintain low temperatures and it's possible there aren't a lot of Dual GPU+CPU use-cases. As a practical matter it simply may not be an issue.
Luxmark has the option to test GPU+CPU. If they ran the GPU+CPU test, that 438W may be the entire thing.