Many cards come with stock voltages higher than necessary.
Yes for other cards that can be more common, but the 7750 is a 7770 that has already been power limited. The odds are stacked against you that there is any headroom on a 7750 that isn't easily controlled by PowerTune.
If you live in the artic or anartic circles and have a very cool environment or otherwise regularly keep your PC in ambient temps below 10 C (50 F) you'll have room to increase your voltage and to a smaller extent you'll have better stability and can thus reduce your voltage but the downward limit change is smaller than the upward limit change in cold ambient temps.
If you live in the hotter parts of the world and your ambient where the PC is is often above 30c (86 F) you'll gain stability by lowering the PowerTune limit and may have more headroom if you lower the voltage on the card but the high heat environment tightens the limits of how low or high the voltage can be making the undervolt option even harder to hit any noticeable power savings vs stock.
Experiment with the PowerTune slider in your hottest environment until you find the highest setting that improves performance (too high a PowerTune setting can actually slow down the video card). Turn it down even further if you want some margin for error, and further still if performance isn't your first priority.
Any tweaking you do will require thorough notes of ambient temps, case temps, GPU temps and lots of stress testing with graphics test that will show you FPS preferably with max, min, and avg values.