I thought you were talking about the rear boxes effecting emissions. You have cats in your manifolds and front pipesThe primary cats are in the headers on my facelift 4200...Both lambdas are situated here...so there is no impedimant to replacing the secondary cats with a a straight through substitute, if thats what you mean..? Emissions are tested at MOT at the tail pipe, as im sure you are aware. If the 2ndCats are deleted, emissions will go up...Im confident in that being fact. The older the cats, the less eficient they will have become too. All part of the puzzle.
Id probably be happier to "pootle" about if I could hear that lovely cross plane crank, which would be good for my licence ! Not so much my neighbours, I fear !
My view is if there is nothing wrong with your cats don't touch them. If they are shot then get Larini Front section because of the quality of the cats they use is superior to the majority of what is freely available to weld in to a custom pipe