Regarding the fans and CPU combinations, I don't have any hard numbers on this, but I think you're right, the Core 2 Duo's were designed to run cooler. I've upgraded R60's, T60's, Z61t's, Z61m's and Z61p's to C2D's and never needed to upgrade the fan. I never noticed the fan working harder or running longer. I recently upgraded my T60p from a T2600 to a T7600 and I think the fan runs less often and/or at lower speeds.
I never upgraded any fans during the process, however, I will say that I've noticed several fan designs for the T60 machines. I'd say I've run across at least four difference fan models where you can see the the layout of the cooling pipes on top of the heat sinks are different. Some are fatter, some wind around more, etc. I didn't make a note part numbers or designs. I do suppose the fans with fatter cooling pipes that wind around the most would cool the most, but testing would be needed to prove it.
The Core Duo T2400 thermal rating is 31w and the Core 2 Duo is 34w. My testing showed that the T7200 did run a little hotter than the T2400. In the T60 specifically, there seem to be four types of fan. 41V9931 and 41W6406 are for Intel integrated graphics. 41V9932 and 41W6407 are for discrete ATI graphics and you have to choose one based on your system configuration. Mine was the discrete graphics version and it had the stock 41V9932 installed. This fan seems to have been used for Core Duo processor configurations. When they configured it with a Core 2 Duo processor, they used the beefier 41W6407 fan. It has slightly fatter and longer pipes. I suspect it was done to cope with the additional heat the cpu throws off.
I decided to upgrade my fan at the same time as I upgraded the cpu but I tested the temps of the cpu and gpu with the old fan and the new one. It did make a noticeable difference and my findings are in this post. viewtopic.php?f=29&t=80730
I don't think it was necessary for me to upgrade my fan - it ran ok with the old one. But I measured lower temperatures with the new fan which meant the bottom of the case was a little cooler which was my goal.
I don't have a T60p and so I cannot comment on the fan situation there. I'm just letting you know what happened with my upgrade so you have room for thought...