You can do a few things. Here is one:Place 5 shelves of SAS 10K on one pair. (node 1/2)
Place 5 shelves of SAS 7.2K on the other pair. (node 3/4)
If you want to "evenly" distribute disks, use the software ownership.
Disable disk auto-assignment
Assign all even disks (0,2,4...22) to node 2 or 4 (depends if it is pair one or pair 2)
Assign all odd disks (1,3,5..23) to node 1 or 3.
Reserve sufficient spares.
Create aggregates on each node. With the disk assignment above, all aggregates could be created the same. (same # of disks, raid groups, etc)
When disks fail, you will have to assign disks as the auto-assign will be disabled and there will be more than one head with disk ownership per shelf.
You should be able to get similar performance on each node. Try to use as many SAS adapters as you can.