Which should tell you something...
Pretty much all hardware drivers do little more than shovel bits from piece of hardware to another. There may be some smart DMA structures to lower CPU overhead, but all in all the vast majority of the performance smarts are inside the chip, with the driver just being a thin layer around it to deploy it.
I believe there are enough performance optimizing tactics deployed in GPU driver that result a significant competitive advantage. If you're in a business where a 10% advantage over your competition can make or break a sale, it's not unreasonable to want to keep this kind of code secret.
It will be interesting to see the results of the ATI open source driver and compare its performance to a similar close source equivalent, but I wouldn't be surprised if the difference will be larger than 30%.