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?
It's not a matter of which GPU model you are using, it's a matter of rig stability. Start with the default (0), but if your rig is not stable (reboot, restart), then try mode 1, which is the same as v1.5.0 (which had somewhat lower performance than v1.6).