I wasn't really brave enough to rinker with the voltages, until yesterday. I was able to get down to 1.38v, also the temps improved a lot; mostly under 85 and one or two cores like 87. Cinebench score also improved by around 150 points (now 4020p).
I also tried higher clocks and got to 4 cores @5.3 and 4 cores @5.2 but temps increased again and throttled.
Yes, I use with an NZXT kraken G12. It's an MSI ventus OC, so it wasn't the best cooler and pretty loud.