So long as you're doing it right, the smoother your surface is, the better.
TIMs are there to fill in the tiny gaps in the surfaces so that there is extremely high thermal contact. The fewer and smaller these gaps are, the less TIM you need.
The only way you could make things worse is to lap the block to a very high grit and then use too much TIM. Even this shouldn't give you performance worse than a poorly lapped block.
Lapping over 600 grit or so will probably do next to nothing for your temps (unless you're such an expert at mounting that such a tiny difference matters), but it won't hurt them.
Alchemy
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