Ok, I get it. But using multiple public IPs as you've described the situation seems irrelevant. As long as your existing wireless router is able to support 100Mbps over its WAN port, that’s all you need. The only possible concern I would have is that since you never get 100% efficiency w/ ethernet, it would be better if the WAN supported gigabit speeds (1000Mbps). But most consumer routers are only 100Mbps over the WAN. A 100Mbps WAN might only get you say 85-90Mbps actual. But for now that’s a minor quibble.
Since you're only trying to increase the *local* wireless throughput, you just need additional APs. Of course, they need to be using different freq/channels (for G, preferably channels 1, 6, and 11). They can either be dedicated standalone APs, or even cheap wireless routers reconfigured as APs (give them a unique static IP and disable their DHCP servers). Just patch them to the router LAN to LAN. No matter how you distribute the wireless access between them, all the router needs to handle is 100Mbps since that’s all the ISP is delivering, and which is far below your router’s maximum capacity (your typical consumer router has 4 x 100Mbps switched ports, so we already know is can handle up to 400Mbps across its switch).
So unless there’s something more complicated here that I’m missing, I’m just not seeing the need to involve multiple IPs and WANs. That gets you into all kinds of other messy problems (e.g., users being on different local networks). You want to avoid that as much as possible.