For what it's worth, I was surprised when I tested lapped vs the milled and anodized finish, I measured no difference. I guess the Arctic MX-4 gets it done!
Have you measured coolant temps?
Before and after picture of the water blocks I picked up for $9 each. They all came shipped in a single bubble bag, banging against each other. I think the lapping is warranted. You can also see the middle block has an inlet that's out of alignment, and the barb nicked by the sand paper.
Measured some temps this eve.
Ambient temp outside the tent: 71.1F
Ambient temp inside tent: 23.3C (73.9F)
Temp of the air after it passes through radiator: 23.5C (74.3F)
Temp of the water loop: 78F
There's some algae starting to grow in the tubes that feed the blocks, naturally.
I'm not a big fan of the arctic silver 5 I bought for this, relative to the generic white thermal grease I usually use. A.S. is harder to wipe down into the near-transparent layer that I get with the grease. *shrug*