I spent quite some time on this topic and this is what I found:
1. The Drobo 5D is quite slow compared to other options out there. It also is somewhat of a black box in terms of their fault tolerance standard. I have had a Drobo 5 FS (GigE) for a few years now and it is painfully slow. Lucky if I see throughput over 20mbps.
2. The two large-scale solutions appear to be either the Pegasus R6 or the Areca. Pegasus comes in a 6 drive configuration and you have the option of ordering 2 or 3TB drives for a total of 12TB or 18TB. The unit does not come unpopulated. The Areca can be purchased as an empty unit.
3. Either of these units will only mount to one machine at a time. So if you have a Mac Pro and a Macbook, simultaneous access will need you to create some kind of share.
4. Throughput on either one appears to be in the 600-650mbps range, which is quite impressive. Certainly faster than a single SSD. This would be in a RAID 0 or 10 config. RAID 5 may be a little slower, especially on write speeds.
5. After the announcement of the new MacPro, I decided to go a different route. I went with external mini-sas enclosures. In an 8 drive array, I get about 900Mbps. I felt mini SAS gave me a few more options and some degree of future-proofing. Performance was also better. My redundancy is also not just at the disk level, but right down the the hardware and chassis.
6. Both WD and Seagate now have NAS specific drives that run much cooler and support various nuances for RAID. This avoids common problems you'd see when using desktop drives, mostly related to the array dropping disks. These new drives are not to be confused with enterprise drives. They cost about 10-15% more than their desktop equivalents, so not too bad.
This, along with the new ATI 7950 card have given my aging mac pro new life. I already had 24GB so memory was not really an issue.
If I do end up with the new Mac Pro, I can always pop the two PCIe RAID controllers driving the Mini SAS arrays into an external thunderbolt housing. Sonnet and Magma already have a few models out. Having said this, TB will bottleneck the current performance I am getting with having the RAID controllers directly on x8 PCIe slots.
My main concern with the new MacPro is that I don't believe TB2 will stand for too long. It really should be called TB1.1. If that happens, then we are stuck with TB2 on the new MacPro. A windows machine may be an option, and will be cheaper, but not sure I want to go there just yet.