Just to clarify, the SSD engine’s single writer has a queue depth of 1, while there are many concurrent readers (currently hardcoded to 64).
Assuming your assumption is “Some workloads will see higher write throughput by deploying more than 1 storage server processes per disk” then yes it is true. We don’t have a lot of experimental data to suggest how to decide what your StorageServer/disk count should be, however. It’s going to depend on the disk and the workload, and also the total processes per machine should probably not exceed the CPU core count. Probably the best advice is to try increasing it and see if you like the results.
Also, while I haven’t verified your quoted StorageServer/disk counts for our example deployments I can tell you we did not choose those with the intent to suggest a specific SS/disk density.