At the moment the config is headers, both cats, then straight pipe, no back box, no middle boxes. This is how it was when I got the car. Would I benefit from fitting a back box?? Apart from reducing the noise would it increase back pressure? By the way its a 97 Sport.

So having posted the same question on FB I've been offered a 3" H&S style backbox, I'll be collecting it on the 23rd, gotta be worth a try  :)

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Man after my own heart

what is the car used for ?

if its atrack day car best route would be to remove ur cats and possibly a side exit exhaust  no back box resulting in no back pressure    

aiding quicker release of exhaust gasses

if the car is used on road replace your existing cats with a high flow sports cat and and put on that h&s back box  to retain /amplify the all important flat 4 burble  it's a good compromise for a road /track application  (key thing make sure the system is running 3inch mandrel bends throughout to eliminate back pressure and to aid accurate exhaust flow ) I hope this has been helpful