A game will be limited by either cpu or gpu.
You have two good parts there.
A GTX1070ti needs a very good quality psu in the 500-550w range.
If your psu is not delivering the requisite power, I would expect to see display artifacts or some other symptom.
Also, an old or cheap psu can deteriorate over time.
Your games are largely single threaded and cpu dependent. I doubt that 4 more threads from a I7 would help.
You said tried oc to 4.2.
Is that what you are running now?
If not, see what happens if you oc a bit.
If you are getting the same fps on low and ultra settings, it indicates a cpu limitation.
More ram never hurts.
But adding ram will give you a decision to make.
If you add 8gb, you may find the addition less than compatible.
Ram is sold in kits for a reason.
A motherboard must manage all the ram using the same specs of voltage, cas and speed.
The internal workings are designed for the capacity of the kit.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards, can be very sensitive to this.
This is more difficult when more sticks are involved.
That is why ram vendors will NOT support ram that is not bought in one kit.
If you do buy more disparate sticks, they must be the same speed, voltage and cas numbers.
Even then your chances of working are less than 100%
What is your plan "B" if the new stick/s do not work?
Sometimes increasing the ram voltage in the bios will make things work.
If you want 16gb, my suggestion is to buy a 2 x 8gb kit that matches your current specs.
Then, try adding in your old 8gb,
If it works, good; you now have extra ram.
If not, sell the old ram or keep it as a spare.