> Can you actually saturate the links with the spinning drives?

There can easily be a bottleneck depending on how the setup the sata/sas, but if you can get sustained sequential reads or writes, 16x drives at 6 Gbps sata should be able to saturate 2x 25 Gbps ethernet. The store link shows two expansion ports as well which should help get bandwidth to the point where 25 Gbps is useful.

Less likely with random reads/writes or mixed use.