Precat removal is always a good place to start. A good 2.5 inch catback system such as Magnex. If you message a chap on here called Daz, he may be able to help you out regarding a nice 2.5 inch custom downpipe. If you do this then you won't have to worry about the precat as Daz will sort that whilst making you your new pipe.
3 pipe mod to the airbox and a good panel filter such as K&N will help out too. And don't forget a 200 Cell Sport cat. The dearest things out of all above will be the 200 cell sport cat and the catback system. You should be able to get a sport cat for under £100 from the internet and the catback system will probaly cost about £350. Don't bother with a Remus straight through center pipe. Its money down the drain.
Hope that helps.
;-)