You won't be able to get your 1060 to 1070 levels of performance, its just to large of a gap for it to make up unfortunately.
What you can get is free performance at the cost of a little time in finding out whats stable, and probably with a little more heat and noise because of this heat.
As for how much, a few %, maybe around 5%? Depends on the card and situation. The good thing about Pascal cards is GPU Boost 3.0, where the card automatically overclocks itself by boosting above the default clock speed when certain parameters are met, basically thermals. You can manually overclock a little more but like I say, it won't give you a massive amount extra.
At 1440p the GTX1060 is certainly capable, but like you have found out you might have to adjust settings down depending on the game if you want to achieve a certain level of fps, 60 for example.
One thing I personally have done is go for G Sync monitors which help smoothen things out even when the FPS varies.