Okay, I promised I'd report on the sound/power changes.
Both are noticeable improvements. The sound is dramatically different. I've put 7000 miles on the car with just the DT Catback, so I know how this car sounds. The headers really changed things. It's actually quieter at cruising speeds and the rasp I would sometimes get around 4500 rpm is basically gone. I would describe the sound as smooth.
It's not boomy or intrusive in the cabin, but it roars when you step on it. I made a little video that gives a sense of it. If anyone knows how to embed this, please do:
As for the power, there is a difference. The dyno shows about a 8-14% gain in torque. I'd say that's about what if feels like. 10% isn't overwhelming but it is noticeable. If I had a Dashawk, I suspect I'd have dropped at least a 10th of my 0-60 time.
I did do a run on the on-ramp from 0-70. Usually when I punch it, the tires chirp a bit before they catch. This time, I heard them rotate on the pavement. It sounded like pulling a single sheet of paper out of a pack and the car slid over a bit before it caught pavement.
So, the sound is fantastic (to my ears). The power increase is there. I wish I could confirm it on the track, but the nearest one is an hour away. Hopefully I'll be able to afford another dyno run after I get my intake replaced.