I need some education from you knowledgeable folks in the world of networking.

What does NAT Throughput mean?

Example: I'm looking to install a multi-WAN router in a location that can only get a few slow connections (but they want to support a large number of users). They're looking to get a few cable modems in order to accomodate the load of all those users.

First, I know that if I have three 10Mbps connections, that doesn't mean one computer could get 30Mbps. It would get 10Mbps. However I assume three computers would all get 10Mbps in this scenario.

What I'm trying to do is figure out a spec on the router that was ordered. The router claims it has an 80Mbps NAT Throughput. What I'm trying to do now is advise the property owner about what he should order from the cable company. Am I correct in assuming that if he pays extra for the 100Mbps connections they offer, it would be a waste of money? Does this just mean that if a single PC were on the network, it would get 80Mbps, but two PCs would get 50 each? Or does this mean that the router can only distribute 80Mbps to the network in total?

Thanks for your wise counsel