Cost to replace catalytic converter for Elise S?

[Hello Paul 36]

Hello,

I've been having a persistent P0420 engine warning code for a few months now. I've had the O2 sensors replaced, but the issue still remains, and from a recent inspection, looks like I probably need a new cat, despite the car having done only 20,000 miles. I am curious how much this should cost me (parts + labour + VAT). I have the 2bular 8" standard 2ZR-FE road rear exhaust box (a pointless piece of kit whose sole purpose seems to be to make long journeys a droney, noisy nightmare). The cost of the replacement cat may or may not be covered under the car's warranty - unbelievably, the garage I spoke to say that the warranty will only decide whether or not they'll pay out AFTER the work's been done.

Anyway, seeing as I'm presumably going to end up getting the thing replaced anyway, is there any mileage in opting for something like a 2bular 2ZR HJS HD Catalyser to OEM manifold (https://www.2bular.co.uk/2zr-fe/2zr-hjs-hd-catalyser-to-oem-manifold) instead of the bog-standard item? Or is this just another noise-inducing thing?

Any advice would be very welcome!

...getting very pissed off that my Lotus is living up to its traditional acronym.