i had a de cat on my last car a 106gti with custom exhaust it was a bit too loud really as the system had no middle box and on a rolling road i has a few bhp down over my last figures duen to lack of back pressure.
not sure how much diff it would make to a 1.2 but you may find your emissions dash light may come on, you will need to put your cat back on for your mot, once i fit my new exhaust system that comes in a few weeks i may still fit a decat to see how it sounds it is a good bhp/throttle response for the money, with every + there are - though, try and pick upn a second hand one and see how you like it.