It depends on the skill of the welder to be honest, as i managed to do mine ok...
Having said that, it was a REALLY crappy bit of welding, as the thin tube wall and rust doesnt give good conductivity. Also they probably dont want to risk it, as if they do blow a hole in it because they have the current turned up too high on the mig plant, then they would be liable to put it right/not charge for their labour.
All in all, a new aftermarket cat back is the best plan, as even if you do manage to sort it now, its only a matter of time until it goes completely, and its not worth paying £100 or so for a welder to have a go, when you would be 1/3 of the way to a milltek/blueflame.