There is a difference between the F2 and the OBX/Focussport headers. One being the primary size. The F2 uses 1 3/4" primaries whereas the others use 1 7/8" primaries. This creates different breathing effects on the motor. The F2 will retain more of the cars around town driveability as the peak torque will arrive earlier. The OBX/Focussport is more helpful with the high end HP generation but will also help flatten the torque curve. So you may have less peak TQ with the same car/same mods with the OBX/Focussport but the torque generation will be much more linear.
I know with my ST I have the F2 header on it. Love it. Retains a lot of the low end power but I gotta say it really falls on its face in the higher RPM. If you look at my dyno sheet you will see both the HP and TQ peak and then drop off after around 5500 RPM. Now one thing you can do to counter this though is install a better breathing intake manifold like the one made by Cosworth. But honestly how many of us are going to buy a 700 dollar piece and then pay for custom tuning for it... lol. I am poor. Thats just me.