Yes it's likely the voltage, but it could be your heatsink mounting as well. Just to be sure - you said "enough thermal paste to do the job." What paste are you using? And exactly how much?
Five important points:
1) ALWAYS USE GOOD QUALITY THERMAL PASTE
2) DON'T use very much at all
3) Have a good spacious case with GOOD air flow and plenty of fans
4) Consider your ambient or room temp might be affecting things
5) Relax - this is supposed to be fun
I use Artic MX-4 (which is wonderful) and only a small dollop about half the size of a pea. Maybe about equal to two BBs. Put it right in the center of the die and then let the heatsink/cooler flatten it out. I agree that the Hyper 212 EVO is fine and you should not be that hot. Post your full system specs.
Although I have a better cooler (Corsair H80i) my i7 4790k only hits about 75C under Intel Burn Test. Don't over tighten (*hugely important), BUT...make sure you are tightening the cooler down enough to get it tightly fitted against the IHS. Sometimes out of fear of breaking something, people are afraid to screw it down enough and there ends up being a small gap (or gaps), causing the entire surface of the IHS to not be making full contact with the water block, or in your case, the cooper plate. That Hyper 212 EVO is a fine cooler, but don't you dare use that crappy Intel stock heatsink. It makes a better paperweight than a cooler.
My idle temps right now (according to RealTemp) are 25/21/25/26. My voltage is at 1.202 right now (it's on "Auto" in BIOS) and I have "Enhanced Turbo" on also in BIOS causing all four cores to run at 4.4GHz - 24/7. Even under heavy load while gaming (BF4, Watchdogs) I'm only getting into the mid to high 40's (C). And my system has been on for about 5 hours this morning.
Take a deep breath, get some MX-4 and start over with the 212 EVO. And get it screwed down nice and snug. Then go outside and throw that POS Intel heatsink as far as you can.
I have a first batch 4790k (L3) and have no temp issues at all. Yes there is a chance you have a bad proc, but its a very small chance. Start from scratch and remount. Then test again.
i7 4790k @ 4.4GHz
Corsair H80i
MSI Gaming Z97M board
Corsair Air 540 (x3 140mm fans - push/pull)
16GB Crucial Ballistix 1600MHz RAM
MSI OC GTX Gaming 770 4GB
Samsung 840 EVO 250GB (boot)
Intel 320 160GB
WD 1TB HDD 7200RPM
HP BD-ROM
PC Power & Cooling Silencer MK III 750 watt
Twin 60Hz - 22" 1920x1080 monitors (one Samsung, one LG)
Windows 8.1 Pro 64 bit