Back in the day lazy machining was the way to undercut your competitor. Nowadays ya can't "get over" with that due to all the testing available online and manufacturers have discovered that a flat surface is not the most thermally conductive. The best performing heatsinks all have a slight curvature on their mating surface and lapping destroys it
Prolimatech does not condone any type of lapping done to the CPU or to heatsink base. Every Prolimatech's heatsink base is designed on a pin-point scale of how the base is to be flat and/or curved where it's needed to be. We have programed our machines to machine the surface in a very calculated way. Any after-manufacture lapping or modding done to the base will alter the design, hence negating its performance factor as well as its warranty
^that is what one site says about one product. Many people have had success with lapping. I guess take a look at the base of the heatsink, if it is nice and shiny then it probably wont help, some heatsinks are quite rough on the surface and wouldnt make good contact and then it may be worthwhile. Also, lapping the thermal plate on the core 2 duo's will likely make it a bit cooler as has been proven many times, there are a few threads/tutorials about this if you use google.
"The best performing heatsinks all have a slight curvature on their mating surface and lapping destroys it"
No, and your response is incorrect. The curve you speak of is on newer CPUs, the edges are rounded off to prevent any uneven contact center-ward. Having complete contact transfers far better, which is why lapping is a good idea if you are an over-clocker or your CPU runs hot. Better conductivity means faster heat disbursement...which is what gets rid of the heat FASTER. OEM CPUs normally do not come pre-lapped, they'd be more expensive if they did and many people wouldn't buy them since the majority of computer users aren't into over-clocking.
I wouldn't bother. Use a quality thermal compound like Arctic Silver 5. Prperly applied, it will fill in any microscopic imperfections between the CPU's heat spreader and the surface of the HSF. I usually tin both surfaces first and wipe the excess off. Then apply as directed. http://www.arcticsilver.com/methods.html