had mine done recently (had a powerflow box from years ago) the ss zorst from cat to backbox cost £120. theres a guy in birmingham (rj performance) his a sound bloke comes very highly recommended!! 2 of my mates told me to go to him.
if you do tell him sharki sent ya with the mk3 gti
when you 1st get your cat back done it will sound very tinny but give it a week or less and it will sound good.
since having mine done its very responsive but thats with a drilled box and a light weight & balanced flywheel

hope that helps