Well, the jetting isn't always gonna be the same. While they may have tried all those pipes on the same bike without changing the jetting, to have the pipes run to their full potential, they very well should've been rejetted or some adjusmtents might have been in order. From my little bit of experience, the FMFs like another size or two on the main from the Toomeys, but every single bike is different and who knows. But just to throw on a set and go, even if it's jetted for a different type of pipe, isn't completely accurate.
My fatties will hang and then some with any comparable T5 setup, dunes, mx, whatever. In fact, let me grab a couple dyno I have comparing the two and a few others. These runs are on a 370 long rod, but you can see the comparison between the two pipes. They're very similar in hp, though the T5 is a little higher and more overrev, but the Fatty torque is stronger.
First is hp, second it torque.