It will be very interesting to see how this goes, as Primitive shaders offer a great chance for devs to offer more performance on AMD, especially if this same feature is on future console iterations like the Xbox One Project Scorpio.
A lot of what Vega is designed to do is create more performance with less, with features like their Draw Stream Binning Rasterizer culling unnecessary geometry for increased performance, their new compute engine designs offering additional throughput and a new memory architecture that can use external memory as well as operate with lower latency.
Add this to AMD's Primitive shaders, where developers can set up profiles to cut down waste on unnecessary workloads and AMD has a huge chance to make gains over their previous architectures.
Sadly Primitive shaders will make no difference in existing games, though it will have a noteworthy impact on games moving forward.