The premise of an arbitrary "sweet spot" combination of CPU/GPU without specific workloads up for consideration is absolute malarkey. Performance and visual quality are separate issues. Any CPU can bottleneck any GPU under the right conditions, and any CPU can saturate any GPU under the right conditions.
Most hardware enthusiasts placate to a heap of mysticism regarding the relationship between CPU's and GPU's. You should match your CPU to your performance goals, and match your GPU to your visual quality goals. Any attempt to match them to each-other based on any area of consideration other than differences in driver/API workload is fundamentally flawed. Ironically, when we do consider the differences in driver/API workload, AMD GPU's should not be paired with AMD CPU's. It is in fact, Nvidia's drivers that scale better into the many-core architecture of AMD CPUs.