The FX-9590 chips are essentially overclocked 8350s. It's entirely possible that there are 8350s that clock with less voltage than it's 9590 counterpart.
Besides, 1.5VCore at 5GHz is the median for 8350s on decent cooling and boards.
I'm hoping to push 4.8 out of this chip without going water. Considering it runs stable and cool at 4.4GHz with 1.36VCore, I'm thinking 4.8GHz should put me around 1.46-1.48VCore.
Oh, and if you're wondering where I'm pulling these numbers from, it's not a hat nor my ass. There's a guy named RGone that has done extensive clocking of these FX 6 and 8 chips. Maybe you've heard of him? Of course you have, you had conversations with him about this very matter(9590/8350) on overclockers.com