I think Candy may have a point. The Milltek cat and DP is a free flow system. To mate that to a relatively restrictive OEM 2 box system may create a back pressure in the Cat, and hence raise the temperature in the cat, and hence raise the stress on the cat.
Would be interested to find out though.
....You're absolutely right, Dave - That's
exactly what can happen and if a component mix did result in a Cat failure, then Milltek would not be obliged to replace it under warranty.
That's why it depends on more precisely what the failure is or how caused, and hence the 'get out' clause.
Apart from trying to save money, IMO it's a fool's game to mix components, especially when a product has been designed and developed as a whole.