it would be as noisy as f*ck, and wouldn't pass an MOT, but I am sure you know this already, so I assume you only want this for off road use.
The engine would run, but you can only know truly how well by trying it. I don't think it would give any real benefit, except improving the departure angle at the back.
some members of this forum have reported improvements by removing the cat and fitting a straight through pipe instead, still retaining the back box. this is a legal mod to the td4 and will still pass the MOT. (I believe)