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.
Thank you! I already have AIO and won't be going full custom loop. Motivation to lap is not to achieve huge gains overall but to reduce hottest cores / core-to-core variance. Information I've come across indicates lapping of 10980XE didn't reduce coolest core any further but hottest ones dropped as much as 6 C.
lapping wont get you far.... its delidding, that does, if your cpu can support deliding.
In my case i even permanently removed the IHS, and am using a Delid IHS Guard for direct die cooling.
But i would not attempt this unless your case handles the board in a horizontal position, so you do not need to clamp the sink down, and i also spent 2 hours lapping the waterblock so its flat as a door knob.
I honestly would not bother lapping the CPU IHS.
Not worth it to lose that warranty should you ever need it for marginal overclocking gains.
I will say that if you do a delid/relid, you're probably voiding the warranty anyway. At that point, lapping the IHS can make sense. That's what I did with my old A10-7700k. It was a lot easier lapping the IHS with it removed from the CPU.
You need sandpaper from about 800 grit to 2000 grit. You also need a flat surface like a pane of glass. You also need a crap ton of elbow grease. If you lap the heatsink you also need to lap the heatspreader or it is a total waste of time. TBH lapping even done right is a waste of time imho.