Please note that Datomic's architecture is substantially different than most traditional triple stores so I don't think it is useful to extrapolate performance for one from the other.
That's true, and if your workload is such that you're able to predictably query over a relatively small fraction of the database, then I can imagine it working well - it effectively buys you auto-partitioning. If your queries run over a larger subsection of the data, you're still going to run into the fundamental problem of most triple stores: they're join and random i/o heavy. I've not heard that Datomic has anything special going on with regards to this issue, although I may be out of date.
This isn't intended to bag on triple stores - I work on them for a living - or Datomic specifically. Triple stores are very useful and in many ways liberating to work with, but they do present challenges when querying over lots of data.
