If your CPU temperatures are being reported by software to be well over 100°C, it is most likely a faulty reading. Most CPUs cut off by the 100°C mark, since anything above that will mean the die overheats to a point it can suffer permanent failure. As a matter of fact, at 478°C, you would notice a heavy burning metal smell from your Mac, as well as smoke, potentially plastic melting on the case or parts of the case, and a CPU that would literally fall off the logic board as that amount of heat is enough to melt most common types of solder. Given your computer was running in a normal fashion while iStat was running, you can be assured it was a faulty reading.