NCT emissions, need exhaust advice please

Ok so D-Day is looming and I can't wait to just get it over and done with! I think the car will have no other issues other then the emissions. I have a cat for it but I have a turbo back straight back exhaust. I probably need a downpipe too. Here are the details of my car:

99 Subaru Impreza 2000 turbo with a few mods. It is over fuelling also. That is aother worry for me to be honest. Is there any other things I may need to look out for in the NCT?

I just changed the down pipe on mine
You can buy stuff called diptane in motor factors wich is stuff you put in your fuel
Tank wich helps to lower emissions for the test

Noway, I never ever heard of that! So you changed the downpipe? Did you already have the cat on it?

The cat is in the original down pipe

i also have straight pipe from turbo,had test bout 3 weeks ago.emissions were'nt much over,co not above 30% mine was 49%...car had been parked for a month or 2 before tat so dont know if tat affected it.lad in nct center said if tat went again now it could pass..!!.im goin for retest soon im goin to chance puttin in some of tat stuff to lower emissions any hope for the best..

dipathane will only clean the injectors and so forth if you have no cat then all the dipathane in the world wont help. Once you get a decent cat in you will be grand . The over fueling will be an issue for sure what makes you suspect this ? id say lambda sensor is gone aswell