The Initial Performance Of NVIDIA's R515 Open-Source Linux GPU Kernel Driver
The open-source Tesseract OpenGL game that isn't too demanding on modern GPUs did see a sizable performance difference on the open-source kernel driver, but again not too terribly surprising considering the early, alpha state of the driver.
In none of the gaming tests was the open driver faster than the default proprietary kernel driver.
Here is a side-by-side look at the game tests where there was a measurable difference between the kernel drivers tested. Games without a statistically significant difference aren't shown in that table - see all the Linux gaming benchmarks via this OpenBenchmarking.org result page that also has NVIDIA R510 reference numbers too.