FYI, Optiminer v1.6.2 was just released.
[1.6.2] Implement second pci mode (--pci-mode 1).
No idea what it is.
According to Optiminer (on the Zcash Mining forum), mode 0 is the same as v1.6.1, while mode 1 is the same as v1.5.0.
I only just starting using optiminer with v1.6.1... so how does choosing mode 0 or mode 1 differ in relationship between using RX4xx and everything else?