That 600mbps is the throughput of the encryption on those devices. It’s no different crossing networks, but the speed will be limited by the network speed. The benefit of a p2p vpn is that you don’t need to shut it off when you join the same network. The devices remain accessible at the same ip whether they are on the same network, or if one is somewhere else. The overhead is negligible and you gain the security isolation that would normally require subnets and a firewall.

In the end, yes, I can stream HD video just fine from another network. For most people, the limitation will be their home ISP’s uplink speed.