While a machine's GUI is closed, the server shouldn't be sending anything to the client about the state of processing going on inside, unless external texture changes are needed, say to show a progress bar. If that's the case, the server should only be sending a small update packet when there's been a change, or when a new player comes into range that doesn't have the correct data. There's specific functions built-in to handle this already and they work wonderfully when used right.
Things like sounds playing and such do require a packet to be sent to start or stop them, but if they are going to play on a loop, that should be coded into the client so the server doesn't have to keep sending a "Start/stop" packet.
EnergyNet calculations should not be running at all on the client. There is nothing visible that happens with the EnergyNet.
I really do hope IC2 goes open source at some point in the future because I'm sure the community will be able to help optimize and streamline it with great success while still letting the IC2 creators control the direction of the mod.