I have a Magnex twin dtm cat-back, and it's pretty quiet. It's only rated at 90dB at 5000rpm. I think the legal limit is 95dB, but when you consider a change of 3dB is double, it makes it pretty quiet!
Nice sounding though when you give it a bootfull of throttle.
Not seen/heard a Milltek on a Mk3 Golf before, so can't comment on them.