I think your biggest issue is that you are recording 16 streams(would guess 8 per drive) that get fragmented all over the place.
Hard drives do not cope well with this kind of recording(very random writes/reads all over). As long as the recording goes well, playback should be just fine when not still recording.
As said above, BD plays fine even at usb 2.0 speeds(i have done some of this my self), but with BD you are simple reading one file(in a simplified way of seeing it). With your setup i assume you are writing 8 streams to each drive so the head has to jump all over to write the files and read them. This leads to instant fragmentation. This fragmentation happens even under windows MCE(this use of a second drive is good)
On to the point, Intel SSD Cache is more made for accelerating files that are often read from the disk. Since you are constantly writing files, you are not going to get much benefit from it. You can use it as a write through(data will fill a portion of the SSD before being written to the hard drive) cache,but i am not sure how well it will work(it may well reduce the fragmentation since it will be writing is large blocks of data and not 8 different files at a time)
One thing worth trying is to cut down on the number of cameras you are running to do some test. Try with half as many and slowly increase until the issue starts. At that point, you will have found the limits for the drives in use. Your only option at that point is to add more drives so lets say 4-5 streams(cameras) per drive.
Depending on the way your system is setup, a SSD will take all the cameras at once, but will wear much faster(would assume you will be filling it every day). Each night, offloading the files to hard drives for storage(since if you watch it will only be one at a time later and should not have issues).