The german tuner 'Hamann' said they would produce a tuning package for the M5 E60 and make it produce 600hp. I haven´t heard anything that the car actually produced that much power or was even finished.
It´s easy to author a press release that says so. But why would BMW have limited the engine to 507hp when a lot more would have been possible?
If it´s about the maximum that´s possivle, a pre-production test-v10 produced ~700hp on an engine test bench, but the engine block could stand it.