For a constant load, as wiredTigerConcurrentWriteTransactions is varied a minimum in performance is seen at a value of 16 concurrent write transactions.
Tests below on 6-CPU VMware machine.
Insert-only workload, 64 clients:
Mixed (but write-heavy) workload of inserts, updates, queries, 512 clients:
Profile and stats showing effect of changing from (effectively) unlimited to limit of 16 during the middle of a run: