In addition, increasing the number of threads also known as concurrency can increase throughput. It wasn't clear what the workoad is, but the FAS system is designed to perform excellent when multiple clients, or applications with multiple threads are sending data requests.
Another good idea is to separate out the iSCSI traffic onto a private VLAN. This is similar to separating Fibre Channel traffic on to a Fibre Channel SAN; except in this case the "Storage Area Network" being used is Gigabit Ethernet. Separating the traffic ensures that consistent, good bandwidth is available for the iSCSI server to FAS connection. It prevents other less improtant networking traffic, for example Desktop users downloading mp3s from interfering with your iSCSI storage traffic.
Sounds like a logical disk setup -- the most obvious way to grow back-end disk performance is just to add another shelf and then grow the aggregate (add another 13 disk RAID group and go to 2 hot spares to keep things balanced).
Before doing that though, if you have Operations Manager in the environment, it comes with Performance Advisor which can show you quite nicely if you're maxing out your disk throughput on the aggregate.