There is another issue with this that hasn't been brought up by anyone else. That issue is how vsync effects the relation between frame rates and refresh rates.
When Vsync is enabled, the game renders, in an ideal situation, the same number of frames as your refresh rates. This prevents the image "tearing" that some find annoying. Now if the frame rate begins to drop below the refresh rate, the game switches to rendering a frame at every other monitor refresh. This is the only way for the game to not begin displaying tearing. If the situation worsens still, the game switches to rendering a frame every third or forth refresh. Going to other direction, with vsync enabled, the frame rate is capped at the refresh rate unless the game can to run at a frame rate that is the refresh rate doubled. At this point though, a frame is being rendered in-between the monitor refreshes and said frame isn't even being displayed on the monitor.
To give an example, if you have your monitor refresh set to 60 Hz. (flicker city) and you are playing a game that can get up to 90FPS on your hardware, you will get frame rates capped at 60FPS. If the scene grows in complexity and your hardware can only render at 50FPS, your frame rate will drop to 30FPS.
Now, with DOOM 3 capped at 60FPS your performance will depend on what your refresh rate is set to when vsync is enabled. If it set to 60 Hz. you will have the previously outlined example. If it is set to 85 Hz. your frame rate will be capped at one half of that or 42.5FPS.
Food for thought.
Brian