Currently there is no support for Upserts to occur in batches, similar to what can be done with InsertBatch.

From testing, throughput for a single update is roughly 1/3 to 1/2 of batched insert throughput. Further, single insert throughput is about 1/3 to 1/2 of batched insert throughput. Extrapolating this, batched update throughput might follow the same curve which would be a great boon to anyone concerned about throughput.