I'm tend to agree. The Slice forums are full of people who's upgraded their Slice to Compute Module 3 and are using everything from glued together bits of copper to crushed copper pipe as a heatsink for a very heavily loaded CM3 in an enclosed environment. Outside of extreme circumstances any thermal dissipation on the Pi CPU makes enough of a difference that the difference between different types wouldn't matter much.
We did test a whole bunch of heatsinks with an IR camera, though, which is what prompted the vent holes at the bottom of the Pibow 3 Coupe (previous holes weren't for air, but rather LEGO). I don't know if we kept any of that data though!
For best cooling, and flair, though, I'd suggest picking up a 5V DC 20mm fan. I offer this advice more because I think they look adorable than for any practical reason, but moving a little air over a 6mm heatsink would most likely net you better cooling than a larger heatsink alone.