You can’t compare the 5500M Apple uses to the “standard” 5500M. It’s a different GPU. Apple uses the full Navi 14 chip (where the regular 5500M has two CUs disabled), most likely binned and probably with additional optimizations. It is going to be way more energy-efficient. The GPU is likely much more power efficient than what you think. Vega Pro 20 too was only 50W.
I should get mine by the end of the month and I’ll give it a test run

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Not much point. The integrated GPUs have already caught up with low-end dedicated ones. Next year Intel iGPUs are supposed to get another 2x boost — and we are getting HBM3 that solves the cost issues of its predecessor.