when you say "exhaust" you are referring to TBE section only, and not the exhaust manifold correct? That was all that was changed?
I'm guessing the TurboXS TBE was not full 3" and not a straight through design on the muffler? If that's true, you should also be seeing gains up top, where there are none to be had. Interesting.
I bet you could really give that 6-7K RPM hp curve a bump with an EWG setup!