a 15% improvement at a trillion dollar scale company is massive

15% improvements are usually called “fixing a mistake in the code” or “getting to that task in the backlog for optimizing that code we had to ship on a deadline”. They’re more likely the bigger the company: more contributors working in disparate areas means more low hanging fruit is probably lying around.

There's nothing ordinary about downloading a new GPU driver and having performance go up by 15%.

This is untrue. It is very ordinary. How much do you know about GPU drivers that you state this so assuredly? Drivers are software. Software can be improved. Do you believe there are no prior examples of GPU drivers being improved such that particular compute patterns go up in performance by more than 15%? This driver improved Total War performance by 71% https://www.nvidia.com/download/driverResults.aspx/74714/en-... Also note you can go ahead and improve any open source driver right now, most likely. Compile it for your specific card and remove all other architecture specific if-cases and you can get an improvement.

Edit: also here’s a opencl 30% compute perf increase documented here : https://m.hexus.net/tech/news/graphics/74425-haswell-systems... that i just googled for

No, we do this kind of thing regularly. Optimizations often depend on the model that is using them. Namely, fusion techniques.