It should mean that if they get the 100Mbps service, 20% will not be usable. You're assuming the 100Mbps will actually be fully available. I'm not sure the difference is enough to worry about in that case. The 80Mbps will be a total figure. So two machines will be 40Mbps each in your example.

Think of it like this. Say the WAN was 1000Mbps, and you had 20 machines - they're not going to get 50Mbps each. The router can't scale up like that.

Also if you have 3 cable modems you'll need a router that supports three WANs which may be a higher model. 80Mbps seems a little slow particularly for something that might be able to handle 3 WAN interfaces.

What are you looking at?