The tape idea seems good, but the only thing that concerns me is conductivity and residual electricity in the chip (if any?) I mean remote possibility but its something.

Lapping should even out temps because your creating an even contact surface for cooling. Because theres 4 cores in different locations you may then see cooling differences based on heatsink/waterblock design. If 1 of the cores was in a bowed location then it will be reflected in its temps.

Goes with a lap that is practical. Making it a mirror finish wont truly net you any real gains, its the making it flat that will. Just hitting it with some 180 to take down to the copper then finishing with some 400 will probably net you 95% of the gains you would have gotten going to 2500. Considering its usually only a 5-8% gain in most scenarios its hardly worth it. going to 2500 is just bragging rights.

Lapping is a really great idea though for people who really are at the edge.