I second what Mikedsilva says, test it open and see if muffler is a issue.
My previous dyno testing (3.0SC small port) has shown that mufflers can kill HP. Dyno below is a car with two glass packs straight off the headers compared to no glasspacks. The Glass packs were straight through but cheap with lots of metal pushed into the exhaust flow.
My research shows that the B&B headers are not bad on stockish 3.6 (i have a set to test). I have a similar 3.6 with DC 993SS cams but stock intake. It's in my 91 964. I have dynoed it in many configurations. I compared my most current dyno results with yours (wondering if I should try your cams). Power and torque were similar to yours until 5K, then mine was higher. Latest dyno below was as above with Steve Wong chip (not optimized for cams and a little lean above 5K) FVD headers and Muffler. The FVD system has 1-5/8 headers (short), 200 cell cats and a 2 in 2 out muffler that should be free flowing (x design). The other test (blue)was stock heat exchangers with cat bypass and secondary muffler bypass.
If you still had stock intake I would question if your resonance flap was working. With ITB's i would expect your 5K plus to be better.
I recently built a new exhaust with 1-3/4 headers that are a little longer. They have merge style collectors and expansion, then into a custom X flow muffler. I need to dyno this configuration and get a custom chip to maximize my A/F ratio and see how it compares.
My previous testing has shown that similar cars tested on different Dynojets show very close results when tested back to back on the same Dynojet
john