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I don't have a definitive answer for you, though spit balling, the S85/S65 are high compression motors 12.0.1. The S62 in the e39 M5 runs lower with 11.0.1.
To make big reliable forced induction power you need to drop compression which takes some extra steps and tuning compared to strapping on a base supercharger kit.
You can run more effective boost with the lower compression ratio, I'd wager that is a large contributing factor. Perhaps also different drivetrain losses between platforms?
It is a shame that you need to spend big money to make a considerable amount of more power with these cars. BMW didn't leave a whole lot left on the table while keeping it N/A and emissions compliant.