I hate to bear the bad news, but as long as the model is too large to fit entirely in VRAM getting 5 t/s on a 8x7b is going to be difficult. You can throw another 16gb RAM in the system which could help with caching and context length, but since the model is still having to juggle data in and out of VRAM the speeds will remain low.
I wouldn't upgrade the CPU personally, focus on adding beefier GPU. And it's probably not worth adding the 970 to the mix, the 4GB isn't providing much room and will likely slow down the 6800 XT more.