I would venture that when you change the fuel delivery and the maximum shutoff point, you will change the HP curve/falloff...I think the max RPM is listed for a stock engine at the point they put merely becuase redline is very near there and thus the IP is shutting things down.

Now the torque, yes I would agree that torque will not remain flat clear to the top however it would be safe to assume that you are widening that band by delivering more fuel at the top/across the band. Sorta like pushing that falloff you feel at 4400 up to about 5200-5300.

I would still execpt a reasonable falloff (hell I can feel it start kicking down at about 5250-5500) but I think the main reason you see the sharp torque falloff is due to the fuel cutoff on an IP that is setup for this to happen...