[04:20:30] [Cypress 0] Error: Device returned hash with difficulty < 1
Unusable for P2Pool, then?
That error means the card found a hash where state_H != 0. While the error should really be "difficulty < 0.9999847..." we just round up to 1.
Because the OpenCL code only returns nonces satisfying state_H == 0, this is most certainly a hardware/OpenCL/kernel problem and not an unusual interaction with P2Pool.