Re: I'm not a fan of Virgin...
My pi setup uses a several year old pi b, and indeed one of the nics is on the USB. I tested it and it can support 40Mbps bidirectional when NATting. As my downstream VDSL is only 16Mbps that's fine. It was not compute-bound, so I suspect this is the USB limiting things.
Haven't looked at the newer models, but a 3B should do a lot better if you use it's wireless NIC for the internal side. Even then, you may not get 200Mbps overall simply because of the real-world performance of 802.11 b/g/n/ac, but a bit more creativity would use one or more of the USB ports to run wired network segments for some stuff and offload the wifi.
ISPs selling 200Mbps or more services should really take a long hard stare at what wifi can actually deliver on the LAN side. It would help with customer complaints.