Regarding Zen performance, a guy who worked for AMD (at least his linkedin profile says that) and who, as he claims, worked on designing L2 cache for Zen and K12 said that their focus was to be competitive against Intel. He no longer works there but apparently his old colleague who still works there said Zen chips have already been tested and so far "it has met all expectation" and they "haven't found any significant bottlenecks". Apparently they haven't finalized the specifications for the clocks and TDP, but their partners in server market are "very excited".
It's not much detail, but I think if there was a problem from having only 2 AGUs, it would count as a significant bottleneck.
Also this is my first post ever, I just usually lurk here and this is the first time I have something useful to add to the discussion. Please no bully.