Im using Premiere Pro for NLE of family events, few weddings and odd jobs. Material comes from digitized 8mm, VHS, HDV and currently 1080p from a 5Dmk2. Its a hobby but I do enjoy working with a system that doesnt slow me down. For video I also use Encore and in future After Effects. For 5DmkII photos I use Lightroom 3 and occasionally Photoshop.
Given this, setting up a full-blown hardware DAS implementation is probably not a good idea, given the costs involved.
You could however, improve matters by using software implementations where applicable, such as a 10 (redundancy of 2 members, and still offers a performance increase).
Granted, the MP has limited internal bays, but it's possible to use an eSATA card and a Port Multiplier based enclosure to get the capacity you need (able to use more than 4x members).
Example kit that would work and looks decent sitting next to your MP.
As per capacity, if you used 3TB disks in all 8 bays, you'd end up with a usable capacity of 12TB, and performance figures at ~400MB/s or so. Not bad, and won't cost you an arm, leg, and first born child.
This solution won't have the robustness of a hardware controller, but it will be a lot easier to deal with. And you can add a second enclosure to the card for backups (JBOD or single disk would be sufficient here, as well as Green disks). You can use Time Machine to manage the backups, but personally, I prefer using 3rd party software due to the ease of use and increased options (scheduling as well as target locations). Check out CCC and Superduper for this, as they're very popular for a reason (also allows you to clone the OS/applications disk to another drive).
If you want a bit more speed, use the SSD's for both OS/applications and scratch on the internal SATA ports that are built into the MP (don't use the same volume for both, use separate disks instead).
Another note, striping disks won't help with random access performance, which is what loading the OS/booting applications relies on.