Thanks for the advice.
I think it may be possible to get to 56Gbps without a switch with a small change to the device firmware files:
56 GbE over a ConnectX-3 VPI adapter
In the end I managed to find some well priced cables from a guy on Ebay:
http://www.mellanox.com/related-docs/prod_cables/DS_FDR_56Gbs_Passive_Copper_Cables.pdf
I only need to connect over short distances, so this is OK for me and gets me started with Infiniband.
As this is for a home lab, I will have to search hard for a reasonably priced switch!
http://www.mellanox.com/related-docs/prod_ib_switch_systems/SX6012.pdf
Thanks again!