The downside of a large development team, of course, is that you have to aggressively move GPUs onto a legacy or reduced support model, and that the support reduction is usually driven not by Linux needs but by the needs of other OSes.
I expect you will see open source driver performance get quite close to proprietary driver performance for most workloads, but for the most shader-intensive or video-memory-intensive applications the proprietary drivers will probably always be faster because they can share development costs across multiple OSes.
Also, if you want to use a multi-GPU system professionally for more than 4 years under Linux, again whether ours or a competitors, you probably should be running on one of the enterprise or LTS distros, and might want to look at the professional SKUs where the support model is a bit different.
The main point to remember is that the open source stack has been relatively stagnant for a decade or so, but in the last couple of years has "come back to life" as programming information for other GPUs has become available and as business interest in Linux use has started to spread from servers to business clients and consumer hardware. There have been very significant improvements over the last couple of years, and I expect that rapid progress to continue for a while.