I would suspect that the current early test versions are not set for speed and, personally, would wait a couple of months before setting up a test disk.
The new graphics API has been available since the first public developer Beta about six weeks ago, seems to be
fully documented, and includes sample code. I presume the kernel changes have also been available since then: it's hard to imagine Apple shipping an OS release to everyone registered in its developer program if the kernel wasn't stable. There have now been four developer releases; I think the end-user Beta tracks the third one. Apple tends to release major OS X revs in pairs. The first introduces major changes in features and the second provides performance enhancements. The kind of performance tweaking I think you're referring to usually comes in the minor releases that follow both types of major release, and tends to be more in the nature of fixing bugs than making significant modifications to the code base.
Given the apparently spotty performance of Lightroom CC/6 with GPU acceleration, I would be surprised if Adobe wasn't at least looking at the new Apple API. I would think the same would go for Phase One.