Has anybody done benchmarks on Ubuntu with the GUI installed versus purely command line? I'm considering adding the GUI to my server, but I am wary of it slowing the system down.
Will you be leaving the GUI running and logged into a session? If you don't actually have the system logged in really shouldn't be much of an impact. The login managers usually don't take up much resources.
You can even the login manager and only start a session when you actually need one. In that case, there really should be no impact other then some disk space being used.
If you plan on actually leaving some GUI applications running that could make a big difference, and it would depend a lot on what you actually left running.
The preferred alternative is to setup a computer you run your management tools from. Then simply use SSH combined with X11 forwarding to run any tools you need on your management station.