Speaking about Battlefield 4, Andersson said creating a Mantle version of the Frostbite 3 engine took them about two months to complete. Mantle support will be added by an update in late December, nearly two months after the game's release.
Jorjen Katsman of Nixxes, the firm porting Thief to the PC, added that AMD's API significantly reduces overhead. The Mantle API has just 8 percent overhead whereas DirectX 11 has an overhead of around 40 percent. Katsman says it's not unrealistic to receive 20 percent extra GPU performance in games with Mantle support.
But the "pink elephant in the room," as he called it, is multi-vendor support. Andersson made it clear that, while it only supports GCN-based GPUs right now, Mantle provides enough abstraction to support other hardware—i.e. future AMD GPUs and competing offerings. In fact, Andersson said that most Mantle functionality can work on most modern GPUs out today. I presume he meant Nvidia ones, though Nvidia's name wasn't explicitly mentioned. In any event, he repeated multiple times that he'd like to see Mantle become a cross-vendor API supported on "all modern GPUs."
Source: The Tech Report