When I was running back and forward, from Kirkcaldy to Edinburgh (before Mum passed), I started off at 12K then put it up to 16K and it made very little difference.
I had to do a wee repair today. The exhaust started wuffling and it was the joint between the cat and the middlebox. The joint is just a spiral sleeve with a clamp on either end. The original clamps had rusted away and someone had replaced them with jubilee clips, which had also rusted away. I fitted a couple of stainless steel clamps which should see the car out. The exhaust system is actually the original, from new. How you can tell is if the middle and rear box are joined by a fixed pipe it is factory fitted. Replacements are joined between the boxes with a second sleeve.