Your are neglecting to factor in the frontal area.
Total calculated drag = frontal area x’s the coefficient of drag.
The TH has a much greater frontal area vs the charger.
Now that my interest is rising, I found a new source for drag and it took some searching but I think I have frontal areas as well.
Frontal area is ~4ft bigger, or 13%.
GC drag = 12.1 (29.5x.41)
HC Drag = 9.8 (25.9x.38)
The GC has 20% more drag, certainly not nothing.
If there is any faith in the dyno numbers, we're showing drivetrain losses that are DOUBLE what the HC has. For example, if your stock HC puts down 645rwhp out of the 707 at the crank... you have 9% loss. When the Trackhawk makes only 575awhp out of the same 707, it's almost 19%. The additional drag of the driveline is causing considerable reduction in power for the amount fuel consumed.
I don't know which is more damaging, but clearly having them together will cause pain at the pump.