my aim was to make a full exhaust system to meet the strick 98dba noise regs of most circuits that my original system failed at 104dba! it also lost performance after burning the baffles out, so an improvement needed to be made. i had to tweak the centre pipe slightly as it had been welded wrong and hit the sump and im sure the corolla one isnt different! i then looked at making a custom manifold back system. my problem was finding a silencer large enough to fit under the back of the car, keep the system a decent length and get the sound down without creating a restriction. well the easiest option was a second hand mongoose back box that was about 2 months old and only £65 
i then made a centre section up to join the it all together using custom exhaust parts and had what i think is a very tidy system.
a support bracket was made to to fit the orginal and reduce he chance of the manifold breaking or cracking. it was then tested around a very wet donny and sounded fantastic without being too noisy.
the performance bellow abut 5000rpm was hardly changed but when up above this figure it would pull very hard showing an indicated 115mph down the back straight in the rain! im sure with a set of cams it would realy bring out some great performance, and a set of itb's would make complete it very nicely.
overall this system has cost me less than a manifold back mongoose exhaust so really has been good value for money as ive sold it on for what i paid for it
next will be a custom 2.5" system for the turbo engine when i have it
if you use demon tweeks you can get 10% imoc discount i believe? but they dont have all the bits in stock sometimes.