The best way to go about solving this problem is by actually profiling different pool sizes. The actualy settings you choose will depend on the nature of I/Os you are performing and of course the concurrency of said I/Os.
Theoretically, should performance
improve with more threads?
SSDs unlike conventional spindle based drives have great random read/write performance and therefore will benefit (greatly) from having multiple concurrent threads perform I/O on them.
How do SSDs handle concurrent writes?
And also concurrent reads, for that matter?
Since there are no moving parts, just data access to various (memory) locations, there is very little queing of random reads/writes that is needed, unless the drive's IOs/s for reads/writes is exceeded.