One with 4GB Elpida memory with best timings available in this forum & speed set to max. 1999 MHz, GPU at 1250 MHz.
The other with 8GB Hynix also with best timings at 2050 MHz & GPU at 1250 too.
Both cards give me the same speed, about 14.5 with 0.41 & 20 with 0.42 (average, not peek).
Blockchain drivers & Windows Server 2008 R2
Ah ok, what I see is my card runs with stock clocks, so 1366 MHZ - that could explain at least a few %. But likely your setup is more energy efficient

Lets see if we can still find a few % for you by writing better code
