(sorry brother this is for OP but replied to wrong person)

At present I have an inferior GPU to yourself (RX480) But I tend to have similar performance on max settings. Occasional drops to 18-20 (this is while not recording) when in the largest rooms.

In an attempt to understand things better i've asked at least 20 different players with different hardware combinations for their FPS ranges and even after a recent graphics pipeline update the game is still incredibly CPU bound. But not clock speed.

Currently I simply can't process a youtube video while playing martyr. The process takes about 3-5 hours. But while playing Diablo 3, I happily does it in the background in about 15 minutes. (Using a Ryzen) It simply eats up every single core I have.

What i'd take from this is that it's undoubtedly your CPU acting as your current bottleneck but could potentially change as optimisation will / should slowly load more onto the GPU - Or so i'm told! The reason for the OC not helping i'd presume is simply because right now core count > clock speed. 

What're your temps getting to in-game on that CPU? I've got a chip in the same series and a higher-end budget GPU and outside of moments of physics spam I don't have a framerate that low, I usually hover around 40 FPS (which is fine, my eyes are bad and I don't generally notice anything in the gap between 30-40 FPS and 120 FPS), but I have a monster of a cooler on my CPU and the Twin Frozr heatsink on my card, so I generally don't break 45c and 70c respectively...

But at overclocking a 4.0 octocore to 4.5 is gonna raise your temps significantly, and depending on your setup that can stray into the "I am so hot I am running poorly now" region.

How much RAM are you running, and what OS are you using?