the sys req's list the recommended GPU as the 970. I got a score around 15.5k with that and a i7-5930k and I KNOW it's the GPU holding me back. If anything, I would go with a GPU upgrade if that 8-core CPU has a clock speed around 3.6 ghz. In fact, if you play other games, I'd advocate going for a higher VRAM GPU as the 970 has only 4 GB and it's not enough to max out GTAV in 1080p (gotta tone some things down). I may post scores with my EVGA GTX 1080 ti FTW 3 GAMING GPU when I install it but I am sure scores will be higher.

In your case, it's probably both the CPU and GPU that can use an upgrade. I don't know the clock speed for your CPU so I can't say for sure.