I'm looking into getting an SA520 but I had a question about the protocol binding feature for use in a dual WAN setup.
If the WAN ports have a dynamic IP:
Send all VLAN 1 (10.1.0.x) Internet (Quad Z's) out WAN1
Send all VLAN 100 (10.100.0.x) Internet (Quad Z's) out WAN2
Yes, that will work. The protocol bindings work to an interface, not to an IP. You can base rules based on protocol, source IP, or destination IP, and then select the interface you want the traffic to use.