I just came across your excellent question, but
I also do not have any hard empirical numbers to share with you.
Possibly, one theory I maintain for the general absence of such numbers
is the reality of quad-core CPUs that are rarely running flat-out at 100% utilization.
The latter means, quite bluntly, that a high-speed idle core is quite capable
of doing RAID parity calculations. Also, our experience has been almost
entirely with RAID-0 arrays -- for speed -- and we have opted for less expensive
RAID controllers that shift some of the overhead to those idle CPU cores.
To illustrate, even an LGA775 CPU like Intel's Q9550 has such a large
amount of on-chip Level 2 cache (12MB), that cache is capable of operating at speeds
equal to, or faster than, the speed of dedicated RAID chips mounted
on Add-On Cards ("AOC"). You could begin your own research by
assembling a Windows software RAID on a more modern multi-core CPU,
and monitor standardized tests with Windows Task Manager. For RAID-0 arrays,
in particular, another factor to consider is the cumulative amount of SSD cache
that results from a RAID-0 array e.g. 4 x Samsung 750 EVO @ 256MB cache
effectively equals 1GB of SSD cache. With such a setup, all that a cheap
RAID AOC needs to do is the PLX-type switching between the edge connector
and 4 such RAID members: the rest of the overhead can be handled easily
by idle CPU cores and the controllers embedded in each member SSD.
Bottom line: at least for RAID-0 arrays of fast SATA-III SSDs, that question
is a little moot. For other modern RAID modes, your question still awaits
comparative empirical analyses. For M.2 RAID arrays, it is now well known
that the limiting factor is the max upstream bandwidth of Intel's DMI 3.0 link.
One observation I noted yesterday was a measurement which showed a RAID-0
of 2 x Samsung 960 Pro running at 90% of its maximum theoretical throughput
e.g. 32 Gbps / 8.125 bits per byte x 0.90 ~= 3,544 MB/second !!!
An aggregate controller overhead less than 10% is quite extraordinary
for 2 x M.2 SSD. However, we won't know how much CPU overhead
is required of RAID-5 and RAID-6 parity calculations until motherboards start supporting
4 x M.2 -or- 4 x U.2 ports that are NOT downstream of Intel's DMI 3.0 link.
