Yep. I have decatted manifolds, EQ front pipes, SS exhaust and did have a 200 cell racing cat in place of the main cats under floor. Apart from the noise, which was nice in a very loud kind of way I had constant engine warning light issues. The fault codes were for the post cat lambdas and when I checked the measurements on Fiatscan they were jumping around as much as the manifold lambdas, even at idle. From what I read up the manifold lambdas should switch from rich to lean to manage the fuelling but the post cat lambdas are just there to check on the proper performance of the cats. If the cats are working properly the post cat measurements should be fairly stable. Mine weren't, hence the engine warning light. I swapped back to a stock cat under the car and haven't had any problems since. Its a bit quieter now but I make the sacrifice for not getting warning lights.