After a lot of cursing and battered knuckles trying to break free the stock cat-back exhaust, I finally gave up and took the car to a local Merlin shop for them to have their way with the bolts on the cat and the second O2 sensor. They did a fine job, everything was mounted up just as I would have done it, so it was worth it to save my knuckles from another night of abuse. The unit starts out at 2.5" at the catalytic converter, expanding to 3" at the resonator, and staying there the whole way back to the 4" tip; I already plan to replace the 2.5" piece of pipe with a full 3" piece (about 8-10" of straight stainless steel piping), welding in the O2 bung and hangars where they should be and attaching a 3" flange for the cat that will be coming from RnR Racing (along with an O2-eliminator downpipe). The muffler is wrapped in carbon fiber with a distinctive "blued" ring around the tip of the exhaust, and has an incredible deep resonating sound; it's a loud unit on the road (especially between 2500 and 3500 RPMs), but completely quiet except for a low grumble at idle. In terms of performance, it feels like it's pulling harder in the upper RPM range, which is the reverse of what I expected (I thought I'd see a lot more low-end torque as a result of the air flowing out faster). I suspect I'll see more dramatic increases from the downpipe. All in all, this was an excellent upgrade, and definitely an attention-grabber (for better or for worse).