beware of the second law of thermodynamics as it can effect C/W. I would say more but second law stuff still makes my head hurt.
I found a thermal distribution map for an old p4
http://www.inqst.com/articles/athlon4/article.htm
Clearly not a uniform temprature distribution.
There is also one for the sun niagara (8 core cpu, low transistor count per core) on
www.aceshardware.com. But im guessing that is of no interest what so ever.