S912's have always run hot so a good heatsink is never a bad thing. I've a VIM2 + heatsink which runs around 60-65ºC so I'd be wondering what add-ons you're using that chew CPU in the background (some do) to keep cores busy.
I am wondering too. I do notice 1 cpu is always using 60% or more all the time when idled. The only add-ons I installed to use is YouTube. Aside from the defaults that was installed with LE, I installed the other media information providers tvdb, screensaver bing photo, lrclyrics, mp3streams and weather. I also uses mysql for my media repository. From that list, I have no idea what is running that is constantly requiring cpu. Screensaver is not enabled and cpu is still 60%+ so i have no idea what errant program that is running. I will probably reflash a clean SDcard and see if CPU is still high to remove any extra thing that is causing overheating.