Whoops, I forgot to update this thread with my results! First off, the A/F readings were definitely screwed up on my baseline run--they had a problem with the sensor.
I just got back from a trip up the coast to San Francisco and back. The exhaust sounds great through tunnels and off cliff walls.
I posted this information (for the most part, I trimmed it for this post) a couple weeks back over on Cayman Club:
I did the work myself at Dynomite Tuning off of Miramar Road in San Diego. Jame there let me use the lift and helped me out when I ran into some installation snags.
I have a friend that refuses to work on German cars, and now I know why. These cars are not easy to work on. Getting the nuts off the pre-cat flange was a pain in the ass, requiring assorted sockets, U-joints and extensions used at non-optimal angles for torque application. The AWE flange is a nice, beefy piece of steel so it isn't flared for strength like the stock pieces. This made accessing the nuts a LOT easier.
So that leads me to my major bit of advice:
When working on a 50,000-mile car, heed the advice of others and use Liquid Wrench, even on a southern california car with no rust.
So yeah, I snapped one of the studs on the pre-cat. Jame had to press it out and we found a bolt that matched perfectly (threads and length and nut diameter, even!) but I'm not sure how permanent of a fix I consider that. I have no idea what the dealership would charge me to press in a new stud, or even if they'd bother--they might just tell me I have to buy a new pre-cat, and that would make me a Sad Panda.
The next little snag I ran into is that the stock exhaust tips don't fit very well onto the AWE Tuning muffler ports. We had to get pretty gorilla on the T-pipe in order to widen it enough to slip over the ends on the new exhaust. I'm not sure if we were supposed to need to do that or if the pipes on the AWE Tuning mufflers are of slightly larger diameter than stock (and I would assume the replacement tips that AWE sells would probably match that enlarged diameter better).
Okay, so with the install out of the way, I gotta say that this exhaust sounds awesome, EXCEPT at low throttle application in the 2000-2500 RPM range. As others have stated, there is resonance/drone. It is LOUD, for sure. It is only annoying when cruising uphill, when the car needs a bit more throttle in order to maintain speed. Otherwise, when you're really accelerating, you pass through this range quickly enough for it to not be an issue.
I expect to be able to get used to it, and if I decide that I'm a big pussy, I'll switch to a Capristo instead.
Anyhow, the sound when accelerating is pure heaven. Everyone that's heard the exhaust says it sounds amazing. Even the girlfriend loves it.
I have a bit more vibration than before, and maybe I'll have to play with the exhaust hanger a bit to see if I can get that to go away.
One other thing I noticed--the muffler cans do not sit perfectly symmetrical next to the tailpipe cutout on the bumper cover. Has anyone else noticed this? On my car, the passenger-side can is sitting about half a centimeter farther to the right than the driver-side one does. I left everything loose when I mounted it all up, played around with fitment as much as possible but I couildn't get any better results. The muffler placement is mostly located by the pipe coming off the pre-cat and that's pretty rigid. It's not something that's really noticeable, so it doesn't bother me, but something I thought I'd mention.
I did this mod for the sound more than anything else, but I did dyno test it, as I was curious to see how it would fare on the smaller motor.
I saw an 8-horsepower gain, which is pretty much what you'd expect.
You'll notice how rich the car runs at WOT through redline--it rides at an A/F ratio of about 12.0. I can imagine how some chip tuning could really release some power. (Ignore the A/F line from the baseline dyno run--the sensor wasn't working right that day. They had it fixed tonight).
At the same time though, perhaps this richening is the ECU's response to detonation, or programmed response to prevent detonation, from the 91-octane gas.
While I didn't gain all that much power (and the butt dyno had already told me that), I was impressed to see torque gains across the board. I had expected to mainly gain power on the top end--the gains down low were a pleasant surprise, and they maintain the smooth and flat torque curve that these cars have, leaving the power delivery characteristics pretty much the same as stock.
I'm looking at about a 15% drivetrain loss and that leads me to believe that I now have about 290 horsepower, up from the 280 stock. Fine by me.