NVDEC HEVC Performance


I’m working in a small startup in VR and I’m having real difficulty to identify the graphics card I need for my purpose. We need to display 8K video @6OFPS in VR headset that are encoded with HEVC (aka H.265). So far we were using a gtx 1070 that was doing the job fine for 5.7K @60FPS video but can’t keep-up with 8K. From the nvidia sdk page : https://developer.nvidia.com/nvidia-video-codec-sdk the decoding framerate is given for several chips and for 1080p decoding. Is it ok to say that if I need to decode 8K, my decoding framerate will be 8x time slower than for 1080p ? Or is it squared and it would be 64x slower ?
I know we could buy a gpu with a TU104 chips, but we need to find the cheapest gpu available than can handle the job.

Thanks for your help !