Yeah, these cards can definitely do 10GbE. On EL6/7 (eg RHEL/CentOS/SL) there is a config file - plain ascii - that you change. In the config file you set the mode for each port.
I don't remember the file location off hand, but you should be able to find it reasonably easily by searching here on the Mellanox Community site.
Be aware that some card models can only be IB or ETH at a given time, not letting you run (say) 1 port in IB mode and the other in ETH mode. I don't remember if the MHGH28-XTC is one of them or not though. You get to find out.