Unfortunately unexpected RAID drop is not an unusual case when the RAID is set with drives, which do not have the TLER (Time Limited Error Recovery) technology. The WD Blacks are great drives for gaming and performance, but talking about RAID configuration they are not highly recommended. Don't get me wrong, they can work well, but it is highly possible to give you errors. Desktop edition drives require longer error recovery time, which may exceed the expected time from the RAID controller, causing the RAID to drop. I presume that your RAID is working fine, if it does not go to sleep, which is totally OK, but if an error occurs and the WD Blacks take a bit longer to recover, you may experience the same RAID drop. I leave you more information about different WD drives.
Talking about Read/Write speeds, the performance may depend of different factors, from the selected RAID type, to the controller and the PC configuration. I would wait for the community to share some experience with RAID5 and see if what you're getting is normal or not.
http://wdc.custhelp.com/app/answers/detail/a_id/1397 – difference between Desktop drives and RAID drives
Hope this helps