might be that the twinscroll is outflowed up top by a single scroll turbo. From what i've noticed of dyno plots between twinscroll and single scroll turbos, is that a twinscroll drops alot more torque to redline than a single scroll turbo.
the engines have no real differences other than the v8 runs a tid bit more compression and cast pistons vs. forged on the v7 motor.
most of the differences will be with the turbo and tune.
this is a v39 that looks really good up top and doesn't drop much torque.
this is my setup.
the dynos are the same but could be using different correction factors and obviously the atmospheric conditions are different
now that i look at it, my car does spool quicker. i'm at 175lbs of torque by 3480 while the other car is at about 130lbs. But, the runs could have started at different rpm speeds and thus the load could have been different.
anyways i know this isn't exactly what your looking for but hopefully it helps, somehow