First: forget about drobo, drobo is not for you, somebody with 12k MacPro needs something better than drobo, drobo is like USB stick on your key chain.
Sans digital are cool, but they only supply software base ready raid cards, despite what website says. Bottom line: they are shipped with esata raid ready card which uses OS raid capabilities, if you will get 8 HD tower going with (FirmTek SeriTek/2ME4E) instead, it will be super rock solid and fast. 400MB/s
But you would need to do some research which esata card would be the best for the job with their case.
You can also go with tested reliable solution from OCW:
http://eshop.macsales.com/item/Other World Computing/MEQX2KIT0GB/
OWC Mercury Elite-AL Pro Qx2 - Enclosure with build in RAID, it will set you 300$. Install 4 x 2TB WD black drives. And you will have very fast very reliable RAID 5 system.(240MB/s)
In my personal opinion this is the most reliable stuff you can get.250MB/S is max what you can get on SATA II with Port Multiplier)
OCW guys have very good support, responsive and very knowledgeable, on the market for many years and providing great MAC solutions.
If you want absolutely the fastest non SSD solution, get FirmTek SeriTek/2ME4E with 2-3 SeriTek/5PM Port Multiplier Enclosures
Check the benchmark:
http://www.firmtek.com/seritek/seritek-2me4-e/perform/
You could go with different card eg.: some highpoint RAID card and have similar results.
Just to compare Drobo speed is about 35-40MB/s on firewire 800.