I think most people know that. The intended takeaway is that this is the only level of optimization the driver has and that it is treating Vega, to the extent possible, as if it were just Fiji.
The apparent lack of tiled rasterization, performance, and efficiency - coupled with the driver...
Beats me, but most likely it's just the original branch upon which the driver is based. The driver installed declares the "Radeon Software Version" as 17.1.1.
Because you can have game compatibility issues, I'd imagine. I suspect this will be enabled on a per-game basis for quite some time before it becomes the primary method and the legacy method is used as a fallback for games which are known to have troubles.