Although I cannot advise you on how to actually monitor the temperature of the dies on the Kentsfield, the LS is a very stable unit in terms of delivering cooling, therefore the best mount/thermal paste/effect of lapping will occur when the evaporator head is at the highest temperature after running for a while. That is, if the evap head reads -45C, the core will most certainly be at lower temperature when the evap head reads -35C (Given the two temperature were taken at equal load levels and at equal ambient conditions.) This is because there is more heat transfer between the dies and the evap head, leading to lower core temperatures, but higher evap temps. Also, keep in mind that the core will be a higher temperature than the evap head, always. (Common sense, yes, but just for the record.)
You can calculate the temperature of the core if you can obtain the exact thermal heat dissipated by the Kentsfeild, the exact amount of heat your LS can remove at peak Operation, the lowest possible temperature your LS can achieve without load, and so forth. However, the Guys at XS will probably be better t this than I am. I can do the math for the thermal calculations, but my knowledge of the evaporation system in the LS runs only so deep.
BTW, good choice with going with the LS. The Mach II GT would probably not do as well, considering the LS is capable of removing much more heat in watts, albeit at a higher temperature than the GT.
Best of luck,