Many good points. I have a few thoughts:
- In many cases 10 gbps interfaces have less than 1 gbps of traffic being sent. Depends on the density of your environment. Of the 20 gbps (bidirectional) available on the trunk you mentioned, how much is actually being used?
- As you noted, Gigamon is one of the primary solutions to scale out SPAN ports. You could also apply an ACL to the SPAN config to drastically cut down on what traffic is actually mirrored.
- Interface bandwidth is always going up, but so are CPU speeds.