I have a motherboard (Asus P8Z77-V Pro / Thunderbolt) and I use the integrated NIC (intel 82579V) to connect to a NAS.
I have seen that the transfer speed is about 20 Mo / sec, so I have decided to enable the jumbo frames (9K) to increase the speed but the speed is still about 20 Mo / sec. The old computer (Code 2 Quad Q6600) was transferring at a speed of about 40-45Mo / sec jumbo frames enabled) (same NAS, switch and network cable).I haev tried other cables and it didn't change anything. The tool from Intel says that the quality of my network cable is very good.
After that I have installed Windows 7x64 (in a VHD) on the P8Z77-V and the jumbo frames are working perfectly (40-45 Mo/sec) to transfer the same file as my test file with Win8.
I have tried a few drives (all the ones I saw from the 17.2 and 17.3 from Asus and Intel and now the 18.0 from Intel) but I can't manage to have the jumbo frames working in Win8x64.