So...the headers have been swapped, and...my car runs pretty poorly.
The primaries on the stainless headers are larger than those on the ceramic ones, and were different enough that the mechanic who did the swap for me had to modify the shield around the alternator for clearance, where my ceramic headers fit without any modification. I'm beginning to think these really might be a one-off, prototype set that were just sitting around DC Sports' warehouse, and they never produced them due to the fitment issues, among other things.
My car (with a cold air intake, NGM Stage IV upper, and an NGM reflash for I/H/E + Stage IV) used to pull from just about any rpm, but pull hard from around 3,400 rpm-ish right to the raised rev limit at 7300 rpm. Now, it feels weaker in the low-mid-range, only starting to pull hard above 4,000 rpm, occasionally stumbles at part throttle or in transitions from part to full throttle, and won't pull past around 6,900 rpm, feeling almost like it's hitting a rev limiter up there. With the larger primaries, I'm not surprised at the softer low-to-mid-range, but the loss of power up high is strange unless it's running really lean up there now.
Now, this may not be all the headers' fault. I also changed the catalytic converter (though the one I put on should flow better than the one it replaced), and reverted back to a different midpipe (from my giant Magnaflow 6" round muffler to the Flowmaster Hushpower HP-2 shorty / long combo I ran previously to gain back a bit of ground clearance).
The cat has to stay, and I'm hesitant to swap back to the old headers. My plan is to first swap back to the Magnaflow mid-pipe, just in case something has gone wrong inside the HP-2's and they are choking things off up high. If that doesn't resolve the high-end power issue, then I'm going to have to get a wideband O2 sensor kit and figure out what's going on with the AFR. If it reveals some significant fueling/timing issues with the new headers, then I'll either have to swap back to the old headers, look at a piggyback (DET3 or SMT8 ) to adjust fuel and timing, or...?
Any ideas what else could cause a loss of power at very high rpms (within 500 rpm of the rev limit), or any other suggestions from some of the more experienced tuners around here (dmdicks, etc.) would be appreciated!