Unfortunately the exhaust shop didn't reconnect the coolant sensor, leading to a poor running, impossible to drive car. The exhaust shop couldn't work out what happened, so I ended up having to get it towed to another garage and fixed.
Here's some shots of the finished product:
And what exhaust post would be complete without a video?
The idle on the car has been sticking on deceleration, which a bit of research indicates the Boost Control Deceleration Device is a likely culprit. I scored a replacement throttle body from eBay for $15:
The BCDD is the valve on the bottom. Three screws later:
And fitted (it's in there, I promise):
That appears to have cured the deceleration issue. I removed the dashpot as well to avoid any interference that it might cause. Unfortunately now the throttle is sticking open a little bit, and I'm kicking myself for not cleaning the throttle body out whilst it was off the car.