I can't help with the car sound but I wouldn't bother putting a filter on a cat. They generally remove their own fur balls and keep themselves fairly clean! It might effect it's "meow" somewhat depending the quality of the filter installed. :D
 
Dont bother. Buy a tank of fuel instead.

However, if you have your heart set on one, then get one. They make it sound a bit more throaty and less whiney (in the 1.2's anyway, dont know about 1.4's).

It made my fuel economy pants too.
 
Had one on my 1.4 for a week while getting replacement Pipercross delivered (it caught fire, long story!)

Gave deep roar under load but not much top end. Did the job for not much cash though. Didnt notice much difference on MPG and dont even think about performance gains! In fact, unless you can get one of those german supercharger kits for the 1.4 dont even think about performance gains on those engines full stop!
 
you'll drop mpg because you'll be trying to decide whether it's a different noise or not