I guess the real question is.... do the 56G cables work with 40GbE. I would assume yes, but because I see this in the Mellanox PDF:
MC2210126-005 Mellanox® passive copper cable, ETH 40GbE, 40Gb/s, QSFP, 5m
MC2207126-004 Mellanox® passive copper cable, VPI, up to 56 Gb/s, QSFP, 4m
MC2206125-007...