i apologize, i read it to fast... 2520 sounds better..
So, if you want HA mode with 12 disks (which is hard)
You will run active/passive setup, (confusing b/c it's really active/active, but all your data will be on one head)
So, Controller A - 8 disks aggr0 1 spare
controller b - raid 4 aggr0 - 2 disks, one spare
There's your 12 disks...
This is a new controller so if you are having problems with setting that up, just give support a call