The 7x00 range do not undervolt the way people think they do. Setting lower mV does not lower the power used, it allows for more headroom on the core clocks as the core is running cooler. The power slider dictates the power budget, not the undervolt.
To use less power while getting more (or the same as stock) performance, you need to lower .mV and also set the power slider into negative.
This gives more core clocks at a lower power budget. Simply moving the power slider to max tells the GPU to use more power regardless of your undervolt.
Edit: In my testing of overclocking and undervolting a 7900 XT.
1. Adjust the power slider until the GPU is using your preferred watts. The AMD overlay will show your GPU power usage.
2. Adjust core clocks, VRAM timings and VRAM clocks to your desired setting.
2. Reduce (undervolt) your .mV until it becomes unstable. Then bump it up a bit until stable and test using actual graphical intensive games rather than synthetic benchmarks.
2. Keep testing for stability in actual games NOT canned benchmarks.
Using the above methods I can overlock with actual 10% extra performance but about 50w extra power requirements. Or my preference is under 300w power used for stock performance.