Hi, Gage,
Thank you for interesting suggestion. Unfortunately no speed-up with trying the OFED distribution - it seems standalone Linux ethernet driver and Ethernet driver from OFED are practically same.
I'm still stuck at 10gbs link-rate and by instrumenting mlx4_en module I can see that query to firmware returns back 10gbps as link speed. I have no Mellanox support contract so have to rely on the third party which sold me ConnectX3 NICs. I'm thinking should I return the NICs now or spend extra 300$ and by Mellanox approved cable. Needless to say that I have IXIA 40gbps tester working flawlessly with the QSFP+ and MPO cables in question.
Regards,
Philip