It has nothing to do with exhaust gases escaping faster. Its solely to keep underbonnet temps down so that the inlet manifold doesnt get heat soaked as much so its mainly the induction side of things.
From experience it works well. I wrapped a 6 branch manifold on a KR 16 Valve converted Mk1 golf that I did a while back and it significantly reduced engine temps in the bay. Ive never heard of it cracking manifolds which is usually down to engine movement i.e buggered engine mounts.