Well well, that CPU is from 2007 ;-D It was not optimized for that CPU.
can you try changing KeccakF-1600-x86-64-shld-gas.s
UseSIMD = 0
You could also mention Core i5-2600K changes in README: 4100 to 8100 khash/s
With "UseSIMD" no effect. Another test:
Xeon E5-1620 (4-core, 3.70GHz with HT):
Ohne path: 7000 khash/s
With path: 7800 khash/s = +11%