Freflow Airfilter and Decat wont gain you a lot other than an MOT failure. Its a chav thing beleiving that just adding a filter will gain you 150BHP.
Filter needs to be done with cold air induction otherwise warm air from the bay will sap what little you gain.
If you have Dual O2 sensors on your car then you'll instantly run into ECU trouble, the ecu will detect that the cat isnt there and refuse to play or start over/under fuelling. Theres also the fact that depending on your car's age it wont go through an MOT. The factory exhaust will be more restrictive than the cat.
The Audi had this done as part of all the other work. The filter and decat got a net gain of about 5Hp. the Audis ECU doesnt actually expect the cat to be there and the original air filter box was used to aleviate warm air problems. The only real net gain was seen afte rthe restrictive (and wrong) 2.3 exhaust got replaced with and Audi Autosport designed one.
Its a mugs game really, if you have lots of money and single figure HP gains will do the job for you then sure. But remeber, cone filters and snorkels dont work well for one (audi is a k&n panel filter) and its not just about HP on a 4x4.
On an injection engine you're better looking at ECU mods, Throttle bodies, remaps and fuel system changes. if its a turbo, look there too.
