It is a per drive limit. The 8TB limit is basically what we can confirm right now as 8TB drives are the biggest drives available right now.
Should we for example get 10TB drives in the near future we will test them as well with our products and then update the product pages accordingly.
For the MB155SP-B you can use 5x 8TB for 40TB total without a problem. So the 16TB RAID 5 is fine as well of course.
I hope this clears things up.