It's 140Gbps for all user spot beams combined, and another 140Gbps if you count the associated gateway spot beams. I guess you could say it's a 280Gbps satellite if you want to count bandwidth the Cisco way (half vs full-duplex) :D

And yes, all of that subscriber traffic will end up at one or multiple terrestrial Internet gateways. One would hope they've provisioned enough infrastructure to scale up to the full satellite capacity. I doubt they purchased 140Gbps worth of Internet bandwidth on Day 1.

User terminals transmit data to the satellite over subscriber spot beams. The data being sent back down from the satellite to the ground station/gateway uses separate gateway spot beams. The 140Gbps number for the ViaSat satellite is for user spot beam capacity, so your statement about half the bandwidth is completely false.

As with any satellite service, there is some form of caching (at the provider/gateway level) along with TCP acceleration. ViaSat is using a very advanced method of both.