1. Can I assume that the grader will have the appropriate logging properties file for whatever they want to see?
2. Would there be any reason my log.info or log.severe statements could fail in the grader's environment.
And knowing that having something in there that is not a requirement can have you lose points, but will never ever ever gain you any points.
So leaving it in or not is your decision. But you are taking a chance of losing points leaving it in.
You say to test the level of non-essential log messages to avoid costly String operations. Did you mean something like:
My opinion is that logging is a great idea, so long as you don't log to a file(just to avoid any File IO issues)