Test Access Ports (TAP) vs. Network Packet Broker (NPB) for Monitoring Software Defined Networks (SDN)

In summary,

TAPs rely on network packet brokers for advanced features; network packet brokers rely on TAPs for bare metal.

Also, NPBs supposedly enable link-layer visibility.

See the following articles for a starting point to getting more info on this topic:
https://www.networkworld.com/article/2903694/network-management/sdn-management-battle-taps-vs-network-packet-brokers.html
https://www.networkworld.com/article/2291059/infrastructure-management/permanent-network-taps-for-complex-infrastructure.html
https://www.networkworld.com/article/2190430/infrastructure-management/network-packet-brokers-increase-visibility-and-performance.html
https://andyhuckridge.com/2012/02/12/maintaining-link-layer-visibility/

How to Calculate TCP Throughput for WAN Links

Ask this guy:
http://bradhedlund.com/2008/12/19/how-to-calculate-tcp-throughput-for-long-distance-links/

His formula is
TCP-Window-Size-in-bits / Latency-in-seconds = Bits-per-second-throughput

This is for a single communication stream. You can max out a WAN link by generating multiple TCP streams.