Have you tried other, more effective methods for reducing group temperature yet? For example, with an exposed group a wet towel will make a significant difference. Setting the pressure lower in the boiler will also make a noticeable difference.
I just tried putting a cold portafilter into my 15 pounds of brass lever group and it took two and a half minutes for the portafilter to feel warm near the ears, with the bottom still feeling cool. Given the small contact area on the ears, it isn't surprising it took so long. So, while it will reduce the temperature of the group, I have doubts about how quickly it will work. I also worry about the effect of the espresso hitting the cold bottom of the portafilter, I know pulling a shot into a cold cup has a negative effect.