I ve fitted one to my polo gti, first you might want to cut the little bit of tubing off (forgot on mine) ,dont think its used on gti, maybe some other fitting(for 1.4?), also you will have to make up a blanking plate for the other side, I also had to lengthen the wiring for the first lambo sensor, and you should/could (up to you

) also wrap the exhaust too,

(mainly due to sound) I know other people will say to reduce engine bay temp etc, you will have to get the flange that comes in the kit welded to the cat, I did the whole job without the bumper removed, but the sump guard you might need to cut a bit, also I had to get the manifold/down bent a bit due to it being a bit to close to the sump, for my liking,
hope that helps
P.s the 'o' ring is the same as vauxhall one,