Bit of an update: Spent a couple of hours at Abbey last Friday having the new Cobra cats fitted. Unfortunately I didn't get chance to take any pictures of them but they looked really well made. Fitting went absolutely fine - no issues with fitting or vibration etc after the fitting + the usual excellent service from Abbey.
Sound
My main reason for getting the HFCs fitted was to improve the sound of the fairly tame Nismo exhaust. I'm not looking for a really loud exhaust sound - I'm 44 and value my relationship with my neighbours so I'm not in the market for something ridiculous - but I do want the car to have a sound that matches its looks. I was hoping to avoid having to shell out for a whole new system, especially because the Nismo system is supposed to be a really good quality bit of kit and I like the way it looks. In that regard, I'd say the HFCs have only been partially successful. The exhaust definitely sounds a little deeper on cold start + there's a difference above 3-4000 RPM. However, I would not say that they've made a big enough difference to the sound to make me not still consider a different exhaust. I will probably just look to choose one that goes well with the new cats. One positive is that I could detect very little rasp from the Nismo + Cobra combination.
Power
I can't say I felt much if any difference but I probably need to spend more time with the new set up before coming to any conclusions. Since I've had less chance to drive the Nismo since we got it than I would have liked, due to the crappy weather, I'm not really familiar enough with the car to notice any subtle differences. Perhaps a remap in future will make a difference.
Overall I don't regret spending the money on this mod. Even though it's not transformed the sound of the car, I have a freer flowing system now, some improvement in sound, and paired with the right exhaust in future it should contribute to getting exactly the sound I'm looking for.