I have to disagree with you there photn, with a simple remap a diesel can get a lot more performance than a petrol because of the turbo.
My car would have a good 30% more power and torque over a standard one, and it uses about 10% less fuel so you really can't go wrong unless you don't like noise.
The exhaust will give improve turbo spin up and maybe a bit more puff at high revs, but if you leave the stock cat then you'll have to put up with this sound every time you back off:
http://www.youtube.com/watch?v=yDk7A1-i0Qo
Believe me the novelty wears off after time..