Decat made a huge difference to my car, definatly worth the hassle of the yearly MOT. The spool up sound alone is just awsome, the scooby burble is properly unlocked with a decat, really deep throaty noise.
As for power, it felt more responsive, and more of a get up and go. Not sure what kind of BHP it gives, Most places say about 25bhp, though tbh i don't think it's half that high.
One thing i can promise you is, that smile you had when you first brought the car, will be twice as big once you go decat lol. I warn you now, you will use more petrol though..... your never want to get out of it. Many times, a quick drive to the shops turns into 1-2hour drive.