Out of curiosity, I performed some testing on hardware that I have access to (all tests are on Windows7 and non-overclocked GPUs).

R7 240 (i=22)  =  9.5%,
6770/5770 (i=20)  =  10.4%,
6970 (i=20)  =  20%,
A10-7870K CPU (i=20)  =  20%,
R9 280X (i=22)  =  84.5%,

Anybody have speed measurements for R9 380 / 380X or AMD Fury?

Thanks