Sonys involment, Thats something we canot know yet. You may as well be right. But as cerny made it sound they have been in more than that. Maybe nothing that would make it a game changer. But could be one or two extra fetures that the competitor might not be allowed to have.
They might have requested some hardware features not originally planned for the Navi chip, which could also explain why there's a 2 year gap since Vega came out until Navi's release. But I very much doubt it.
It's more that AMD didn't have the manpower to develop both CPU and GPU at full speed (which is a well documented fact), and concentrated on the new Zen CPU cores and chiplet designs instead of GCN GPU upgrades.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454