The power limit adjustment in AMD Wattman or MSI Afterburner should be thought of as a theoretical maximum power. Ergo, 0% on the slider means it will never exceed the manufacturers baseline maximum voltage. At 25%, it can scale up to 125% of the baseline maximum voltage, so on and so forth.
Exactly what algorithm determines how much power to use, and when, is not something I've found. However it's unlikely the driver would ever decide to crank up the voltage all the way to 150% of baseline, even if you allowed it with your overclocking software. Even then, should it ever decide to use 150%, it would be under only the most demanding circumstances and likely for an extremely short amount of time.
Personally, for my Sapphire Nitro+ 4GB...