Most copper CPU blocks are lapped but they're lapped for the mirror finish, just because it looks nice and I guess it reduces the microscopic valleys. Even after lapping most blocks will be convex, again by design. Companies like EK, XSPC, Swiftech, etc have their blocks lapped. Mmmmm shiny.
So we should tell a HSF company what to do... Well instead of telling a company what to do instead I asked a company (Swiftech) what they actually do. I asked Bryan from Swiftech about the matter, here's what he had to say:
Being that it was Gabe who pioneered the convex surface of CPU copper heat sinks and water blocks, I can confirm that this is the optimal design for the surface of such devices. This design was arrived at by Gabe's own testing and it has been proven to be the best design for achieving optimal contact between the IHS and the copper block.
Being that when you install a CPU, particularly an Intel CPU, the tension created by the mounting mechanism will cause the surface of the IHS to bow. This is why having a convex shaped copper block is so beneficial to the performance of the cooling apparatus.
I hope this resolves your dispute.
If after all this you don't want to believe these surfaces should be somewhat convex then I give up. Also you will have fun with even more RMA's in the future when you continue to receive surfaces from different companies that are convex.
Edit: Interesting to note, I never knew Gabe Rouchon (CEO, Swiftech) was the pioneer of the convex surface for coolers. Edited by M3TAl - 11/19/13 at 6:41pm