A Blitz Nur-Spec catback will give you 37.3bhp
No, I havent got a clue what sort of BHP you'll be putting out but it should have upped your boost to about 1 bar- quicker and harder spooling turbos are the main bonus.
Get one, or preferably, both of the cats out for the same gain again. It'll sound meatier as well. You'll need a fuel cut defender (VFCC) if you want to boost over ~1 Bar. Do a search.
A boost gauge would also be a good investment to keep an eye on things, especially if you de-cat. J-Specs can overboost.
Losing the cats will make way more of a difference than a filter. After market induction kits tend to filter badly and suck in hot air from the engine bay - the stock one is fine but quiet.