Look at Mushroom Networks. I looked into them when we were researching bonding cheap internet connections to merge them into a 'fat pipe' for vpn traffic. And that phrase brings up another option to look into - Fat Pipe.
Keep in mind all of these hardware elements cost money. If you amortize them over time (2-3 years), it may be better to just get more bandwidth, and "spill over" or "load balance" on a Sonicwall.