These drastic temperature changes will stress the die and package considerabley. Although not "shocking" the material properties of the package are not very forgiving to below zero temperatures coupled with rapid rises in core temperatures. This will create a significant stress gradient that will significantly reduce the life of the package. I think it would be safer to use a heater to bring the die, unpowered, to a certain temperature (say 30C) while the system powers up. This way the package and the core could be cooled simultaneously. Just my 2c.
