Verde, glad you got the CEL issue all squared away. While I was reading your post I didn't think the O2 sensors were the issue (guessing). I did my own install of headers and cats and I know I didn't take as much care of the sensors as you described. I tugged on mine pretty firmly to get the extra slack to reach and didn't have a problem.

Enjoy the new mods!

 

Thanks. I guess I felt it was the O2 sensors because there is a history of 200 cell cats throwing CEL's due to O2 sensor issues. Just seemed like the 'weakest link' even though I did to try to take care. Oh well.
BTW, I have had a chance to take the car for a longer ride. Still not enough to fully program the ECU (I suspect) or give it a thorough evaluation, but it is a significant improvement. By far the biggest performance change I've made to the car.
In short the car is definitely louder but just modestly so. A bit harsh at startup, but smoothing out after the enriched sequence is over. It's got a bit more of an edge and is 'breathier' than before. At cruise at 70 or so, there's no drone and it's not loud at all. But at mid and WOT it really opens up. Louder and a bit metallic. Very very cool.
Re performance, it is definitely improved. As I noted before, it's a logical lightened flywheel. The engine spins up more quickly and engine braking is less pronounced. My 'personal' dyno say that acceleration is improved and a quick check of mileage is improved as well.
Overall, the Fabspeed x-pipe is a marked improvement, and although not inexpensive, it's the price/performance leader for NA cars. To be fair, I did simultaneously change out the headers, but I suspect (from reading others entries) that the majority of the perceived changed can be attributed to the new cats.
I went with the x-pipe because I've been very impressed with Fabspeed's mufflers, and I'm a believer in the x-pipe design (a little venturi effect applied to exhaust flow) but I'm sure similar effects are delivered with AWE or others. Anyway you cut it, I'd highly recommend it.
Oh and do it yourself. It's kinda fun.