Found the info on the Hennessey site:
http://www.hennesseymotorsports.com/
(On the site, drill down to the M5 page, then click "introduction")
Note that at the time he did not disable the DSC which would not let him test above 5600 RPM - still these numbers suggest the Japanese graph is not accurate.
The following is a direct quote from the site. You can go there to see the graph.
"Peak Rear Wheel HP: 317 RW HP @ 5600 rpm (almost 1000 rpm below the factory advertised peak power of 6600 rpm)"
"Peak Rear Wheel Torque: 320 RW LB-FT @ 3000 rpm with over 300 RW LB-FT available from 1800 to 5200 rpm - an incredibly wide and powerful power-band."
"Since the M5's computer prematurely limits max rpm at 5600 on the dyno, we can interpolate the data measured and estimate that the motor produces close to 345 rear wheel hp at 6600 rpm. This equates to what a 400 flywheel hp motor should produce at the rear wheels."
/Greg