RB, if you can afford the cards then definitely go with the ConnectX-3 cards. ConnectX-2 and 3 are the only cards confirmed to work with Windows 2012/8 and SMB3, but - and it's a big but - ConnectX-3 is I believe the only part that fully supports RDMA with SMB3, and that's where your speed is going to come from. That info is based on a blog post on Technet by José (Google it, I'm sure you'll find it).
There has been some success on 2012/8 with the OFED packages and older Infinihost III adapters (such as the ones GodAtum and myself are using, see the Open Fabric Alliance forum) but no working package installer and no RDMA support, just SRP initiator and IPoIB.
I'm leaning away from InfiniBand at the moment. I'll use what I've got for an SRP target on a Linux VM, but that's it. I would have liked to add a switch and a couple more cards and use it for SMB shares but the performance is only 10-20MB/s better than 1GbE. With SMB3, NIC teaming becomes massively more useful as it should utilise all ports even in a single user scenario, so that's what I'll be pursuing since it's much more cost effective at the moment.
. Thanks for the info.