Interesting to revisit my post from this long back as an estimate to the S5, S7 and future Bitfury chip efficiency.  

When the S5 was released, x=25 for Jan '15,

11.2e^(-0.118)(25)=0.586 W/GHs
7.0376e^(-0.112)(25)=0.428 W/GHs

When the S7 was released, x=32 for August '15:

11.2e^(-0.118)(32)=0.257 W/GHs
7.0376e^(-0.112)(32)=0.195 W/GHs

Let's say we see Bitfury chips by April '16, or x=40

11.2e^(-0.118)(40)=0.0998 W/GHs
7.0376e^(-0.112)(40)=0.0798 W/GHs

Could be on to something here, first formula seems to hold somewhat true at this time.