A decat has its ups and downs....
Ups: Nice sound, Few pops and bangs etc etc
Downs: Wont pass MOT, IMO there is no point in doing it to any engine size under a 1.8 (Everyone to there own though)
The only BHP gains is 3-5 horses and about 6 donkeys. You can only really notice a proper decat sounds at around 2500rpm - 6000rpm but personally i would just much rather get a sports cat. 
You've pulled this one out of the bag completely, blindfolded as well.
Where did you get 3-5 hp gain? Little did you know that on the Mk3 the cat is actually very free flowing and non restrictive. Up until 200hp it's good enough, along with the standard exhaust manifold. If you don't believe me ask Glen, he's the one who's tested it back to back on a dyno, and he's running just under 200hp on plenum in his valver.
And you can't compare decatting a 1l micra and a 6.3l v12 merc for example. Gains will be completely different depending on vehicle, condition of old cat.
Nail - head.
I noticed my 1.6 MK3 being about a billion times faster after a decat. Old cat was completely blocked.
TBH mate, I wouldn't bother decatting it. I made my own decat and only really did it because the downpipe broke. The downpipe is part of the cat so it's either a big expense for a custom made thing or hope you're good with a welder.

The worry is there for me now though as Lakeside in Essex have mobile VOSA stations that test emissions etc and SOMEHOW I wasn't one they randomly checked and waved me past. I was bricking it.
Don't bother. Decent backbox and delete the middle box is very loud or just get a nice sounding backbox.
I have a stainless VR6 exhaust catback and sounds nice, not droney and nice deep tone. Don't bother mate, especially as it isn't straight forward.
