There is a point where higher is too high, but I'm not sure where that point is.
When I was into car audio, we used to use fans that were intended for computer applications in our amp racks. It's not uncommon for high-output car systems to see 14v-18v with some of the HO alternators. I had 4 small fans cooling an amp rack and my system voltage could get close to 14v. I don't remember any high-failure rates with any of the "computer" fans we used in those applications, but those fans also didn't stay on 24/7 like computers often do.
So my best guess is that ~15v will "work" in the short term. However, I would consider it an experiment and watch the overvolted fan(s) for signs of stress (funny smells and/our sounds.) I would also expect a shortened life span.