Very cool. As a stop-gap, one can always pipe to a program() to do the actual inserts. That gives you a chance to batch the logs as a TSV and then run mongoimport on the TSV for high-performance inserts. You should be able to get around 20k-50k inserts/sec that way.
I plan to do bulk inserts within the driver - much like how the sql driver does bulk commits with explicit-commit turned on. The plan is to make a writer thread, which will combine a (configurable) set of documents and insert them in bulk. I'm halfway through implementing that, should be done during the weekend.
The key thing to know when profiling MongoDB inserts is that you need to let everything run long enough for Mongo to fill RAM to capacity so that it is forced to begin using disk. Up until that point, everything is done in RAM, which means you're not seeing the long-term rates, only the burst rates.
Oh, that's nice to know, thanks!