Hi, I recently had to replace my CAT on my 01 Astra Bertone 2.2 Coupe and back box, I couldn’t afford the cat from the dealer so I got a universal which is ¼ the price and with the extra cash I had I got a sports back box. The thing is, the car has lost power as in its lost its boost , its just not pushing me into the seat as it did before. I’m not as sure as I was when it comes to over taking, its takes that little bit longer and it now takes 12sec from 0-60 where it should be around 8. Is that because of the cat or the back box ,ie lack of back pressure . I could really need some help on this. dunno..