There is not a lot of information out there about viewing the network interface throughput in real-time via CLI on Nexenta out there on the internet, so I hope this helps someone.

I discovered you can view the throughput in real-time by using a flow control monitor, and flowadm.

First, enter expert mode:
option expert_mode=1
!bash
Hit (y) to confirm,

flowadm add-flow -l NETWORK_INTERFACE -a transport=tcp flow1
flowadm show-flow -S

Example:

flowadm add-flow -l igb0 -a transport=tcp flow1
flowadm show-flow -S

This will give you a nice real-time status of your network throughput.

To remove the flow monitor, simply type in:

flowadm remove-flow -l network_interface
EX:  flowadm remove-flow -l igb0

Please be aware that there is some overhead involved in using flowadm for this purpose, as demonstrated by @kdavyd: