Odd performance numbers comparing 2.0.15 and 2.1+

I've been doing some basic performance testing of batch commitlog on 2.0.15, 2.1-head, and 2.2.-head. I have been seeing very odd results where 2.0.15 performance is incredibly high, suspiciously so, when compared to 2.1 or 2.2.
Example graph:
http://cstar.datastax.com/graph?stats=2ea11748-0562-11e5-9045-42010af0688f&metric=op_rate&operation=1_write&smoothing=1&show_aggregates=true&xmin=0&xmax=1570.36&ymin=0&ymax=127509.8

I'm using the following yaml options:

commitlog_sync: batch
commitlog_sync_batch_window_in_ms: 50
commitlog_sync_period_in_ms: null
concurrent_writes: 64

I am able to reproduce this on two separate clusters, physical hardware, with SSDs. I have not been able to reproduce this on gce, or a physical cluster I have access to with HDD. I can reproduce this consistently, with different stress options, and with higher data load on the nodes. I can also reproduce this myself on these machines, without using the C* perf tool to set up the cluster and perform the operations.

When running 2.0.15 batch commitlog vs periodic, I do see a difference as expected:
http://cstar.datastax.com/graph?stats=4ae77aac-056f-11e5-8344-42010af0688f&metric=op_rate&operation=1_write&smoothing=1&show_aggregates=true&xmin=0&xmax=68.09&ymin=0&ymax=166966.8

I am using the newest stress code on trunk. What should I be looking for to explain the anomalous performance here?