you have to have a valid mot to validate your insurance, technically de cats don't meet mot requirements with the new laws as you need to maintain the same level of kit as was there on manufacture. Racing cats are legal, as they are still cats and pass emissions and still legal. De cats are sold as not road legal, have to removed for mot's unless you use a dodgy/friendly garage. An insurer could if they wanted use it as an area to disallow payment. But unlikely. They insurer named parts on the proviso that the meet the mot regs etc. Its highly unlikely to be a point of contention but there is a slim chance, especially if they are not declared to the insurer.