Performance measurement... a world within a world!
Dealing with Mellanox InfiniBand for the last decade++, I know for fact that it can do more. it is just a matter of use case.
debugging complains about performance can go many directions and can be either simple or very involved.
The community site has few examples of other users who already worked on performance tuning issues. please search.
Given IB is a different ballgame then 1G Eth, few things to consider:
* Server: driving 56g/s doesn't come from nowhere. strong server with capable PCI bus is a key. see this post: Infiniband Performance Analysis?
* network: you didn't mention which switch are you using, type, how many, is the network blocking?
* versions of OFED stack - with IB you can run data on few protocols including native RDMA (best performance), ipoib (which is what you are doing) and few other protocols to mention. you can gain best performance for ipoib with the latest mellanox ofed version 2.1 (available on Mellanox.com)
and finally, checkout the following Performance Tuning Guide: http://www.mellanox.com/related-docs/prod_software/Performance_Tuning_Guide_for_Mellanox_Network_Adapters.pdf with lots of good advises for improving your performance.
Good luck. Please keep us posted on how things look.