I have noticed, that adding more GPUs to my rig, lowers the sols per GPU.
3 x 470s = 15 sols per instance.
5 x 470s = 13 sols per instance.
sometime I only got 7 per gpu/instance
sometime it was 15 per gpu/instance
and sometime 19 per/gpu/instance
using 5x RX470-4G per rig