I had decoupled the pod and activated the heatshield in orbit around the Mun, Kerbin periapsis was ~10km. I used time warp to get from the Mun to Kerbin's atmosphere (not sure what warp setting, but I did use physical warp at one point). I just tried another flight, AP was 100km and PE was 20km, the heatshield glowed but never gave an effect. I'll try another flight without using time warp. Edit: I did several more flights, at varying speeds and angles, with and without time warp, and I still can't get any effects to show up other than the glowing heat shield. It still looks nice.