AMD needs 6-8 core APUs, thats where Mantle can gain most of CPU performance. But they need some time to get there, with new APU platform and new features AMD needs to make sure everything works and improves performance. As they will not lose their ground in low end systems, but in few years they will gain middle end users and in 3-4 years we might see some high end APU.
And they have Opetrons. They have 16 cores and they will need an update at some time.