After two weeks of work I have a comparison ready between our new renderer running with OpenGL and Vulkan:
In a test of rendering performance with 110,000 instanced boxes we saw 80% faster performance with Vulkan over OpenGL. The primary bottleneck was data being piped over the PCI bridge into an SSBO.
Thanks to all the people who offered their advice on this board as I was starting.
Well, they are both using identical techniques, I did everything I could to make the renderer fast, but Vulkan seems to have removed some inefficiency in the driver code.