Another update - which may or may not be useful for other forum members in the future.
Before I placed my order for the de-cat pipes, I contacted Larini, and asked them the following:
Two quick questions if I may:
1) if I went with the de-cat pipes, do you know whether the removal of the secondary cats would lead to emissions failure from an MOT perspective? I understand that all the lambdas are in the primary cats, so no check engine lights, but I’m presuming the secondary cats are there for a reason?
2) is there a reason why on your website it says the de-cat pipes are for off-road use only? I would understand that if they were being fitted to a car with no catalytic converter, but am unclear why this is given they are designed for a car with the US spec manifold with built in cat....
Below is their response:
Hi Jon,
No problem.
1) In the UK you need one set of functioning cats fitted by law, which you have in the manifolds. The primary cats will see you through the MOT, for emissions too - provided they haven't fallen apart (which is rare on the primary cats, but could happen if the car backfires badly for example). The secondary cats were designed for the emission strict regions like California and Switzerland, but found their way on to other GS's presumably due to economies of scale and availability. I have seen both manifold/cat types on UK GS's and it seems to be a bit of a lottery regards whats fitted, bizarrely.
(FYI, it's not shown on our website but our secondary sports cats are fitted with lambda ports and blanks, so if your primary cats did ever fall apart you can move the probes downstream to the Larini cats to pick up a reading).
2) Emissions requirements vary from country to country and in the USA from state to state, where sometimes any removal of factory fit emissions equipment is illegal. Hence we simply add that line as a disclaimer and a warning to the customer to check their local state controls before purchasing.
Hope that helps.