Thicker goop is good for non-flat surface mating.
As already pointed out, goop is better than air and even mirror-finishes have enough air between them that goop (applied correctly) can lower the thermal resistance.
- If surfaces are super-flat (regardless of the finish quality) you need a less viscous thermal compound to provide a thinner layer between the two surfaces.
- If surfaces are not particularly flat you need something stickier to act as a bulking agent and hold the compound in place where the two non-flat surfaces don't make direct contact.
If you think your IHS and waterblock aren't particularly flat and you don't want to spend ages sanding them flat, then I'd just suggest
more thermal compound; The difference between near-perfect, mirror-smooth surfaces using a thin layer of runny paste and uneven, coarse surfaces with thick goop (but no air bubbles) should be minimal - like 3-5 degrees at most. There's an old old test on Dan's data where he tested the difference between pastes and obscure stuff like toothpaste and vegemite spread. Other sites have since copied him to humourous effect, but the answer is all the same -
properly applied paste is more important than
what paste you use
Admittedly there's a lot of information on how to apply paste without actually discussing whether that's the best way or quantity to apply for any given combination of surfaces. Viscous paste is worse for smooth flat surfaces, very runny paste will not effectively fill the gaps in uneven surfaces, The correct amount of paste is going to involve a little trial and error, looking at the contact patch it makes, but the answer is "as little paste as possible to make contact with the entire IHS". As an engineer with a prior background in fluid dynamics, casting and injection moulding my education says that the ideal way to apply paste is in a small 'X' shape that fills the middle ninth (centre box if you imagine a tic-tac-toe grid on the IHS). That's the ideal theoretical method and in reality the paste application tests show that to do well, sometimes beaten by "pea" or "grain" because in practice it's not possible to attach the heatsink 100% vertically and 100% in the correct position on first contact. When in doubt, just press down and slide it around and fraction to aid the spreading!