Isn't it more about the number of cycles it takes to generate the hash more than it's size, since you're likely to do that very often in a db context?

Probably. I'd venture to say the reason we use SHA is due to its uniform distribution over its speed (or lack thereof.)

I guess it's worth investigating, murmur has been in use by some big names (cassandra, elastic search, hadoop, etc...) for a while in similar contexts.

I was pretty sure (checked almost 2 years ago) that murmur3 had a uniform distribution, too. And in terms of speed is a relatively easy gain