My other reply got moved, but long story short the CPU magically resurrected itself after I installed a new RAM module and reseated the old one.
As far as temps go, my results seem to be not that great. It seems like at best, the lapped CPU is 1 to 2 degrees cooler at idle and load for a given ambient case temperature. This is despite the fact that my IHS was definitely warped (the difference is most apparent when applying thermal compound, as I use the "edge of a credit-card" technique, and it used to be nearly impossible to get an even coat, but now it spreads out perfectly with no extra effort being necessary). Also (and this could be just because I'm testing during warmer times of the day than usual), my ambient temperature appears to be 2 to 3 degrees higher now than it used to be (despite having 4 120mm case fans), so I've actually attained a net increase in temeratures.
Is there anything that might be done to improve things further? Lapping with > 600 grit, if I can ever find a place that sells it?