Just found a link off some American Scooby forum to some dudes site for gutting his cat on an Sti. Reckons it improves power:

http://www.mattrandolph.com/wrx/gutted%2Ddownpipe/

 

Bit too much work on an Impreza IMO (what with welding it up and all). Also when it comes to MOT time you'll need it again. Best to buy a decent decat downpipe with an open neck (so you'll get even more improvement) and keep the std one for MOT time.

I have gutted cats on gti-r's but it's a lot easier as it's just a short cat pipe. No need to drill the cat guts either, just hammer and bar through the middle a couple of times and it soon collapses.