The "Power control" setting on AMD cards is indeed not the same as voltage tweaking. Like you said, it changes the amount of power the GPU will accept from the PSU. Some games and applications (particularly Furmark...) can be so computationally demanding that they cause a graphics card to exceed its TDP and overheat even at stock clock settings, causing crashes or even damaging your graphics card and system in general. Modern GPUs are designed to detect this excessive power draw and overheating and automatically downclock to prevent it. This downclocking, of course, hurts actual graphics performance. By raising the "Power Control" setting, you are raising the power draw/temperature level at which the downclocking kicks in. For games that don't push your graphics card like that, you won't see much benefit, but some games, like Crysis, I believe, will see a performance benefit. DAI may be one of the games that benefits. As long as your PC and graphics card have decent cooling, I don't think there's much risk to it. Just don't push the power control setting to max and run Furmark.
At least, that's my understanding of the whole thing.