There's no safe undervolting I'd be comfortable with when putting on a high load on the GPU when using this GPU, which it sounds like you want to do.
The point is whether or not you accept there's a significant risk. Nothing may happen. But if something does happen, you could be out a GPU, and this would be something you broke, not a warranty issue.
Honestly, if it's something so crucial that you'd risk an expensive GPU on, it's something worth running by a store and getting something safe and dependable.