I have been thinking about this all day and its bugging me... I have looked at all the dyno charts I have here for the F's... I find a few things very interesting... #1 The torque curve is absolutely the WORST curve I have ever seen, it should be flat. #2 The headers gain only 5 HP all the way up to 5200 rpm ish... That seems really odd because you would think the power increase would be linear..
At what rpm does the F get "on cam"? Just like a turbo car, the more agressive the cam profile, the more important your exhaust path is. IE, you are going to be seeing larger increases when the car is on the more agressive cam profile.
Also, the torque curves are similar, it's just the different scaling on the graphs that is exagerating the differences. You can see two small peaks in the torque curve on your DD dynno, but the two peaks on the other graph are much more visable due due the differences in scaling.