I understand that you want to know if the SlingCatcher will increase the Internet usage when installed.
You can hook up the Sling Catcher and the Slingbox on the same network, but this network must avoid any contact with the Internet service (isolated network) Why? Because the Slingbox will try to contact our servers. This situation will increase the Internet usage/charge.
Keep in mind that both the Slingbox and SlingCatcher must be connected to the same router. As long as you follow these instructions, you will not be affected.