When toms was still doing performance charts they said you should try to jump at least 3 tiers to see a notable performance gain. Unless there are extenuating circumstances. When you are already running a 1070 2 tiers would put you into Titan XPP territory. For the cost, I think you are better off holding out for the next gen GPUs. Even then I wouldn't hold your breath too much. I think the next gen GPUs will not provide a notable performance boost. I think AMD is lining up a sizable perf/dollar boost potentially we'll see 1070 performance in a sub $300 GPU from AMD. I don't expect that from nVidia.