i think the undervolting, turbo disabling and clocking apps are the way to go for limiting cpu heat/speed.
i have no idea what you will be disabling when you cut the core count ? is it real cores or HT cores? and how that relationship works out in the end. (will it be 2 real 2 HT cores or 4 HT cores or what?)
id gess the only time it gets relay hot is when your doing real work which is when you actually need the speed so im not shore of the point, apple care (or like cover) for the laptop and use it hard sounds better to me than buying a top end model and cutting it back by 4 cores.