You might see a few C temp drop. Really depends on which HSF you have, what kind of finish it has from the factory, and some other factors.
Lapping heatsinks was "a thing" a long while ago thanks to curved IHS surfaces and heatsink surfaces. People were lapping them both flat. The last time I had a lapped HSF was with my NH-D14 which I did use extensively with lapped CPUs, and it seemed to help a little.
Today, I don't think the concave/convex IHS thing is a problem anymore, so lapping isn't anywhere near as popular. There are still a few reasons to do so, such as if you are using liquid metal and you want to get incredibly tight bondline thickness between your IHS and HSF. If you lap both down to copper, it's like using JB weld in there. They get stuck together. At least that was my experience with CLU. If you are using conventional paste though, like MX-4, then I'm not sure it's really worth the effort. If you do a great job on both IHS and HSF AND you manage to avoid killing your CPU with metal shavings, you may knock off a few C here and there. But if I were running something like a 10920X I would be looking at watercooling of some kind instead.