For clarification, if I set my lens to f/8 and then added a 2x tele-converter, would I have to change the setting to f/4 or could I just leave it at f/8 even though the effective f/stop would be f/16?
If you are using a hand meter that indicates f/8 for the correct exposure, you would need to change the lens setting to f/4 in order to have an effective f/8 result.
If you are using an in-camera meter, and an "automatic" tele-converter, the in-camera meter will most likely deal with the issue for you.
Matt