Zotac, and every other manufacturer, is going to tell you that a unit which is probably much more than what's needed is required, just to be sure they cover their end and would rather give you an overkill recommendation than have you get a unit that's the exact amount needed but that with a cheap unit that doesn't meet spec, won't be enough. You'll be fine with the unit you have. Here's the RealHardtechX recommendations based on their testing. Their recommendations are always already beyond what's actually needed, for the same reason, so there's no need for a bigger unit unless you want one.
Plus, that VX is probably more capable of providing it's labeled power output than that VS unit is anyhow. Like I said, if you DO want a bigger...