derspi has explained all there is. here's my take - i run my T5 w/out a cat, but then i only have 1 O2 sensor unlike N American cars which have 2. take a peek underneath, you shd see 1 wire connected to the sensor before the cat, and 1 after. cars in my region only have the sensor PRE-cat ie. there's no reading for the ECU after the cat, so removal of cat has no direct effect on ECU air-fuel programming on my car.

yes, removal of cat will result in exos drone, you will definitely need a good silencer to take away the excess noise.

and yes, cat removal made a good difference on my car ... initially. the moment i took it off, i felt like i had 15hp extra! but just like the wastegate rod-twisting saga, i think i "lost" the added response over a matter of time. it's the ECU learning again ...

rgds

jeff from Malaysia (no emission control regs here!)

95 855T5A SAM chip, full S/sprint exos, no cat