That's response time. I was referring to vertical refresh rate which is measured in Hertz (Hz). 50, 60, 120 and 144 are typical refresh rates nowadays. Enabling Vsync should automatically limit the FPS to your monitor's native refresh rate, but like I said some games have customizable FPS limiters and some softwares such as MSI Afterburner RivaTuner can also get the job done.
If you're getting below ~95% GPU usage with FPS capped to 50 with only Vsync enabled then it means that your monitor has 50Hz refresh rate and so it can only display that many images in a second and therefore your GPU doesn't need to work at its full potential. If however, the GPU isn't working at full capacity AND your FPS doesn't meet the quota, then you've got a potential issue on your hands which can be caused by either unoptimized video drivers or a physical hardware limitation.
As to your original question. I guess the MSI GTX 670 PE is simply such a beastly card that it can take 99% usage while staying at relatively low temperatures and keeping the fan speeds low, which leads me to suspect that you haven't tapped into its full potential by overclocking it yourself. Also, 99% usage ingame doesn't mean that the card is being stressed to its absolute maximum. There are programs specifically designed for torture testing your card to see how it will perform in ultimate conditions.
For now just enjoy your high framerates and keep your Vsync on to avoid tearing and stressing your graphics card too much.