Tha max you want to run the Quad on air is 1.5v. Anything more I'd have H20 on it.
Your safe in the 1.4 area.
And dude, I'd reapply thermal paste, and see where that takes you. But, I myself think you just won't be able to OC on the stock cooler. They just are not meant to take the extra heat your OC chip is putting out.
But clean off the old gunk with alcohol, then reapply some new stuff. There are a ton of different methods to applying paste, I myself just use a grain of rice sized dot in the middle of the chip. It's always worked for me.