Graphics hardware architectures continue to evolve towards performance that benefits rendering pipelines of virtual reality (and augmented reality). VR introduces requirements that weren't so important for traditional video gaming. Latency and dropped frames (where rendering a frame takes longer than the refresh rate) took a back seat to high-fidelity AAA rendering capabilities. VR needs to render each frame in time and do it twice: once for each eye. Driven by the requirements of this emerging industry, semiconductor and hardware manufacturers are building new and improved devices, which will inevitably impact how content developers think about optimization.
That said, most likely you should develop and optimize for the lower specs that you want to target. If such optimizations necessitate undesirable compromises, consider separate versions...