The M5 was tested in the french Magazine Sport Auto and it yields 619 HP on the DM Performance Superflow Windyn 4WD Dyno (for 600 HP manufacturer)... For comparison the Porsche GT2-RS was measured at 715 HP (for 700 HP manufacturer), the McLaren 720S at 735 HP (for 720 HP manufacturer).
IMO best way to measure HP is with the same dyno, using different cars so you can have a direct comparison...
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