It may be coarse, but you want some surface texture to increase contact surface area. I read an article some time ago from a guy that was mating quartz surfaces to metal heat sinks that said that his lab tested different grits of sandpaper for preparing mating surfaces, and 800 grit was optimal for the silica-based TIM that they were using. Lapping to higher grits flattened out the surfaces so much that contact area was reduced more than was mean bondline thickness, reducing thermal transfer.
Of course, when dealing with one of the metal pastes, surface coarseness may cease to be of issue due to the way it behaves (or the way I think it behaves), and lapping may be of no real value. But going from the nickel surface to a 400-grit lapped copper surface will improve things. 800 grit might be better, if you're using a conventional paste. Mirror finishes are basically pointless from a heat transfer PoV.
Watch this space for future . . . uh . . . stuff! Yeah. That.