The current API for updateWithDamages does not include the record to be modified, so the engine has to re-read it in order to apply the damages locally and write the new value. The read is going to be in memory because the caller had to read it first, but it's still a non-trivial amount of CPU work.

We should just pass down a RecordData constructed from the base pointer and objsize of the target BSONObj. Unit tests can be modified to fit this API without a lot of churn.